We used to require exact match between img_len and raw_len for
non-interlaced PNGs, but the PNG in issue #276 has extra bytes
(all zeros) at the end of the compressed DEFLATE stream.

The PNG spec doesn't have anything to say about it (that I
can tell), and if libpng accepts this, who are we to judge.

Fixes issue #276.

Conflicts:
	stb_truetype.h

so you don't get the automatic info you normally do

My guideline for the rules is the PNG loader (which I consider
"canonical"). In the _load functions, x and y are required but
comp is optional; in the _info functions, all three are optional.

Fixes issue #411 (and other related, unreported issues).