load_ora: load png layers faster, use less memory

load_ora: load png layers faster, use less memory

Use libpng directly (fastpng.hpp) as we already did for saving.
No longer load the full png into a gdk_pixbuf, but load and
convert one tile row at a time.

This should use less temporary memory, especially if lots of
tiles are empty, because we can free them again early.

This should fix https://gna.org/bugs/index.php?17568

Also remove code to recognize solid backgrounds (because it expects
the png to be in a gdk_pixbuf). Rely on the background_tile attribute
instead.

load_ora benchmark: 2.75 --> 2.08