XFA: Support PNG format in pdfium_test and add image diffing
Lays the groundwork for pixel-diffing tests in pdfium.
This is a port of chromium's tools/image_diff/image_diff_png.cc onto the
top of the fxcodec-provided version of libpng. pdfium_test is modified
to support the new format, and the tools/image_diff.cc image comparison
utility is provided, stripped of its base/ dependencies.
Unfortunately, this can't be back-ported to origin/main, since the
underlying PNG support isn't present inside pdfium.
BUG=https://code.google.com/p/pdfium/issues/detail?id=62
R=thestig@chromium.org
Review URL: https://codereview.chromium.org/870203005
6 files changed