PNG: Ignore invalid gamma chunks

[brianpopow]

Prerequisites

• I have written a descriptive pull-request title
• I have verified that there are no overlapping pull-requests open
• I have verified that I am following the existing coding patterns and practice as demonstrated in the repository. These follow strict Stylecop rules 👮.
• I have provided test coverage for my change (where applicable)

Description

This PR handles another edge case when the gamma chunk does not contain enough data.

[JimBobSquarePants]

What does libpng do in this situation? Gamma chunk is not mandatory and can use default values in certain situations.

http://www.libpng.org/pub/png/spec/1.2/PNG-Chunks.html

[brianpopow]

What does libpng do in this situation? Gamma chunk is not mandatory and can use default values in certain situations.

http://www.libpng.org/pub/png/spec/1.2/PNG-Chunks.html

I think you are right, its not mandatory.

libpng only shows a warning, but decodes the image.

libpng output:

Click to expand!

Testing libpng version 1.6.38.git with zlib version 1.2.11

libpng version 1.6.38.git
Copyright (c) 2018-2020 Cosmin Truta
Copyright (c) 1998-2002,2004,2006-2018 Glenn Randers-Pehrson
Copyright (c) 1996-1997 Andreas Dilger
Copyright (c) 1995-1996 Guy Eric Schalnat, Group 42, Inc.
library (10638): libpng version 1.6.38.git
pngtest (10638): libpng version 1.6.38.git

Testing length_gama.png:
length_gama.png: libpng warning: gAMA: invalid

length_gama.png: 1 libpng warnings found
Files length_gama.png and pngout.png are different
Was length_gama.png written with the same maximum IDAT chunk size (8192 bytes),
filtering heuristic (libpng default), compression level (zlib default),
and zlib version (1.2.11)?

length_gama.png: libpng warning: gAMA: invalid

Pass 0: rwrwrwrwrwrwrwrwrwrwrwrwrwrwrwrwrwrwrwrwrwrwrwrwrwrwrwrwrwrw
rwrw
length_gama.png: 2 libpng warnings found
Files length_gama.png and pngout.png are different

length_gama.png: libpng warning: gAMA: invalid

length_gama.png: 3 libpng warnings found
Files length_gama.png and pngout.png are different

PASS (4 zero samples)
libpng passes test
Default limits:
width_max = 1000000
height_max = 1000000
cache_max = 1000
malloc_max = 8000000

[JimBobSquarePants]

LGTM 👍