Tests: Allow to use IMGFLO_TESTS_VERBOSE envvar to increase verbosity
Tests: Add Photoshop reference tests for gradientmap
More consistent sRGB handling

Still had a minor discrepancy to original Photoshop references,
but hoping the perceptual similarity is acceptable now.
HACK: sRGB property inside gradientmap blend now defaults to true,
while it should really reflect setting passed in from outside.