Remove texture sampling from GrConfigConversionEffect

Elevates default precision for the entire fragment program
to maintain precision on some ES implementations.

Re-land fixed version of:
  https://skia-review.googlesource.com/10026/
  https://skia-review.googlesource.com/10056

BUG=skia:

Change-Id: Ibe43dba9c223e430b2f9f8ed5cb97ed7a049e6c2
Reviewed-on: https://skia-review.googlesource.com/10160
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
3 files changed