Modify matrix convolution GM for sRGB testing

Tag bitmap as sRGB, and adjust gain/bias in color-managed configs to
produce results that are less blown out.

Also added a colorized version of the GM, to validate that gamut
conversion is working.

BUG=skia:

Change-Id: I333988dcdaa1272121e8aa731b4188c942fe19d8
Reviewed-on: https://skia-review.googlesource.com/6466
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
1 file changed