Allow FPs to elevate default precision for the entire fragment program

Allow FPs to elevate default precision for the entire fragment program

Currently, GrConfigConversionEffect is able to round-trip on many mobile
GPUs because it uses highp for all intermediate variables (including the
texture fetch result). Separating the texture sample into a different
processor breaks that.

This is a blunt instrument, not to be used lightly.

Bug: skia:
Change-Id: I2ab365e3da79628069e2eb727c43c2bf45bfd789
Reviewed-on: https://skia-review.googlesource.com/10162
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>