doc: More work on the python backends docs.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>
tests: loaders: Add basic tests for SavePNG().

Signed-off-by: Cyril Hrubis <metan@ucw.cz>
loaders: PNG: Fix errno propagation.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>