[graphite] Set up SDF text fragment shader.

Adds the code and sampler setup to support signed distance field text.

Bug: skia:13118
Bug: b/238756869
Change-Id: I57236e0fed305dadfad6d3f757392490fe1e1495
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/561836
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
3 files changed