Make a dedicated (non-runtime) FP for turning on high-precision mode

I'm about to convert GrConfigConversionEffect to GrSkSLFP, and this
seemed like a better solution that adding special flags used in one
spot.

Change-Id: I6720411e9915e76c8f2c75d2b99ca746ceaaa597
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/423516
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
6 files changed