let's like, chill out about all these rules, man

There's really no reason to prevent any of these conversions;
they all have somewhat reasonable behavior:

   - converting between grey in different color spaces
     should probably work just fine
   - we'll convert color to gray using a fixed set of
     luminance coefficients, but that's better than failing
   - we'll invent {r,g,b} = {0,0,0} if we convert alpha
     to something with color
   - converting to opaque formats without thinking about
     premul/unpremul is probably fine, better than just
     not working at all
   - a missing src color space can always be assumed to be sRGB

Updates to ReadPixelsTest:
   - skip more supported test cases in test_conversion(),
     each with a TODO
   - conversions from non-opaque to opaque should now work
   - conversion from A8 to non-A8 should sometimes now
     work on GPUs, and the test needed a little bit of
     a tweak to not expect A8 to carry around color somehow.

Updates to SRGBReadWritePixelsTest:
   - writing untagged pixels shouldn't fail anymore;
     instead, it should behave like it was tagged sRGB

Change-Id: I19e78f3a6c89ef74fbcbc985d3fbd77fa984b1c2
Reviewed-on: https://skia-review.googlesource.com/147815
Commit-Queue: Mike Klein <mtklein@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
4 files changed