[PR 125] Add test for three color types of PNG with tRNS chunk

From: Mateusz Loskot (mateusz_at_[hidden])
Date: 2019-07-18 18:56:47

Hi All,

I have picked up an this old issue, found suitable test samples in the PNG test
suite and updated the PNG I/O tests accordingly. The tests should also serve as
a good documentation of current PNG I/O behaviour for PNG images with tRNS chunks.

Please, have a look, review and let's get it approvedi, and merged.

Best regards,
-- 
Mateusz Loskot, http://mateusz.loskot.net
Fingerprint=C081 EA1B 4AFB 7C19 38BA  9C88 928D 7C2A BB2A C1F2