Improve GrGLNonlinearColorSpaceXformEffect to support scRGB

Improve GrGLNonlinearColorSpaceXformEffect to correctly implement
scRGB transfer function and allow for negative colors.
Relax SkSurface_Gpu::Valid to allow kRGBA_half_GrPixelConfig
render target without a color space needed for legacy blending on
Android.

Bug: b/62347704
Change-Id: Ibc6144e69c26cdbdcbf29348c4f612fb6b639e01
Reviewed-on: https://skia-review.googlesource.com/26143
Commit-Queue: Stan Iliev <stani@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
5 files changed