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.

This reverts commit dffe9827b18444d1426859d9035f9f0087201f44.

Bug: skia:
Change-Id: I940af3256c47e6672a008d516db9e55669672ca3
Reviewed-on: https://skia-review.googlesource.com/11345
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
12 files changed