Increase precision in GrSRGBEffect

Allows ApplyGammaTest to pass on Bifrost, which uses fp16 for mediump.

Change-Id: Icd04001939a2cb7e1789132ab035ca498c92112a
Reviewed-on: https://skia-review.googlesource.com/118162
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
1 file changed