tiff: add support for SamplesPerPixel tag in tiff_decode_tag()

tiff: add support for SamplesPerPixel tag in tiff_decode_tag()

Format detection and internal frame initialization is moved to a
separate init_image() function, which is called when all the tags have
been read, and so both BitsPerSample and SamplesPerPixel information
has been collected.

This fixes decoding of the file 11.tiff from roundup issue #1925.

Based on a patch by Kostya Shishkov <kostya.shishkov@gmail.com>.

Signed-off-by: Diego Biurrun <diego@biurrun.de>