do not assume pHYs chunk is available in QPngHandler

do not assume pHYs chunk is available in QPngHandler

read benchmark results:
********* Start testing of tst_QImageReader *********
Config: Using QTest library 4.13.0, Katie 4.13.0
PASS  : tst_QImageReader::initTestCase()
RESULT   : tst_QImageReader::readImage():"/home/smil3y/katie/tests/benchmarks/gui/image/qimagereader//images/bench.kat":
     0.460 msecs per iteration (total: 230, iterations: 500)
RESULT   : tst_QImageReader::readImage():"/home/smil3y/katie/tests/benchmarks/gui/image/qimagereader//images/bench-large.kat":
     12.35 msecs per iteration (total: 6,177, iterations: 500)
RESULT   : tst_QImageReader::readImage():"/home/smil3y/katie/tests/benchmarks/gui/image/qimagereader//images/bench-transparent.kat":
     4.630 msecs per iteration (total: 2,315, iterations: 500)
RESULT   : tst_QImageReader::readImage():"/home/smil3y/katie/tests/benchmarks/gui/image/qimagereader//images/bench.png":
     1.69 msecs per iteration (total: 845, iterations: 500)
RESULT   : tst_QImageReader::readImage():"/home/smil3y/katie/tests/benchmarks/gui/image/qimagereader//images/bench-large.png":
     21.014 msecs per iteration (total: 10,507, iterations: 500)
RESULT   : tst_QImageReader::readImage():"/home/smil3y/katie/tests/benchmarks/gui/image/qimagereader//images/bench-transparent.png":
     11.17 msecs per iteration (total: 5,585, iterations: 500)
RESULT   : tst_QImageReader::readImage():"/home/smil3y/katie/tests/benchmarks/gui/image/qimagereader//images/bench.ppm":
     0.756 msecs per iteration (total: 378, iterations: 500)
RESULT   : tst_QImageReader::readImage():"/home/smil3y/katie/tests/benchmarks/gui/image/qimagereader//images/bench-large.ppm":
     2.958 msecs per iteration (total: 1,479, iterations: 500)
RESULT   : tst_QImageReader::readImage():"/home/smil3y/katie/tests/benchmarks/gui/image/qimagereader//images/bench-transparent.ppm":
     2.664 msecs per iteration (total: 1,332, iterations: 500)
RESULT   : tst_QImageReader::readImage():"/home/smil3y/katie/tests/benchmarks/gui/image/qimagereader//images/bench.svg":
     10.77 msecs per iteration (total: 5,385, iterations: 500)
RESULT   : tst_QImageReader::readImage():"/home/smil3y/katie/tests/benchmarks/gui/image/qimagereader//images/bench.xpm":
     8.344 msecs per iteration (total: 4,172, iterations: 500)
RESULT   : tst_QImageReader::readImage():"/home/smil3y/katie/tests/benchmarks/gui/image/qimagereader//images/bench-large.xpm":
     76.738 msecs per iteration (total: 38,369, iterations: 500)
RESULT   : tst_QImageReader::readImage():"/home/smil3y/katie/tests/benchmarks/gui/image/qimagereader//images/bench-transparent.xpm":
     65.280 msecs per iteration (total: 32,640, iterations: 500)
PASS  : tst_QImageReader::readImage()
RESULT   : tst_QImageReader::setScaledSize():"/home/smil3y/katie/tests/benchmarks/gui/image/qimagereader//images/bench.kat":
     1.98 msecs per iteration (total: 990, iterations: 500)
RESULT   : tst_QImageReader::setScaledSize():"/home/smil3y/katie/tests/benchmarks/gui/image/qimagereader//images/bench-large.kat":
     13.31 msecs per iteration (total: 6,657, iterations: 500)
RESULT   : tst_QImageReader::setScaledSize():"/home/smil3y/katie/tests/benchmarks/gui/image/qimagereader//images/bench-transparent.kat":
     5.590 msecs per iteration (total: 2,795, iterations: 500)
RESULT   : tst_QImageReader::setScaledSize():"/home/smil3y/katie/tests/benchmarks/gui/image/qimagereader//images/bench.png":
     3.218 msecs per iteration (total: 1,609, iterations: 500)
RESULT   : tst_QImageReader::setScaledSize():"/home/smil3y/katie/tests/benchmarks/gui/image/qimagereader//images/bench-large.png":
     23.224 msecs per iteration (total: 11,612, iterations: 500)
RESULT   : tst_QImageReader::setScaledSize():"/home/smil3y/katie/tests/benchmarks/gui/image/qimagereader//images/bench-transparent.png":
     12.72 msecs per iteration (total: 6,362, iterations: 500)
RESULT   : tst_QImageReader::setScaledSize():"/home/smil3y/katie/tests/benchmarks/gui/image/qimagereader//images/bench.ppm":
     1.64 msecs per iteration (total: 822, iterations: 500)
RESULT   : tst_QImageReader::setScaledSize():"/home/smil3y/katie/tests/benchmarks/gui/image/qimagereader//images/bench-large.ppm":
     3.898 msecs per iteration (total: 1,949, iterations: 500)
RESULT   : tst_QImageReader::setScaledSize():"/home/smil3y/katie/tests/benchmarks/gui/image/qimagereader//images/bench-transparent.ppm":
     3.630 msecs per iteration (total: 1,815, iterations: 500)
RESULT   : tst_QImageReader::setScaledSize():"/home/smil3y/katie/tests/benchmarks/gui/image/qimagereader//images/bench.svg":
     9.304 msecs per iteration (total: 4,652, iterations: 500)
RESULT   : tst_QImageReader::setScaledSize():"/home/smil3y/katie/tests/benchmarks/gui/image/qimagereader//images/bench.xpm":
     9.858 msecs per iteration (total: 4,929, iterations: 500)
RESULT   : tst_QImageReader::setScaledSize():"/home/smil3y/katie/tests/benchmarks/gui/image/qimagereader//images/bench-large.xpm":
     77.880 msecs per iteration (total: 38,940, iterations: 500)
RESULT   : tst_QImageReader::setScaledSize():"/home/smil3y/katie/tests/benchmarks/gui/image/qimagereader//images/bench-transparent.xpm":
     66.030 msecs per iteration (total: 33,015, iterations: 500)
PASS  : tst_QImageReader::setScaledSize()
PASS  : tst_QImageReader::cleanupTestCase()
Totals: 4 passed, 0 failed, 0 skipped
********* Finished testing of tst_QImageReader *********

write benchmark results:
********* Start testing of tst_QImageWriter *********
Config: Using QTest library 4.13.0, Katie 4.13.0
PASS  : tst_QImageWriter::initTestCase()
RESULT   : tst_QImageWriter::writeImage():"kat":
     2.450 msecs per iteration (total: 1,225, iterations: 500)
RESULT   : tst_QImageWriter::writeImage():"png":
     16.28 msecs per iteration (total: 8,142, iterations: 500)
RESULT   : tst_QImageWriter::writeImage():"ppm":
     7.176 msecs per iteration (total: 3,588, iterations: 500)
PASS  : tst_QImageWriter::writeImage()
PASS  : tst_QImageWriter::cleanupTestCase()
Totals: 3 passed, 0 failed, 0 skipped
********* Finished testing of tst_QImageWriter *********

Signed-off-by: Ivailo Monev <xakepa10@gmail.com>