[graphite] Hook up gamma adjustment for SDF text.

We had the shader support but weren't setting the variable.

Bug: b/324265719
Change-Id: I75afd9897d66e4e06a5d62d4ce60b03787269c06
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/853598
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
3 files changed