Remove SkBlendMode from SkSL

We've lived without this for a long time. It bloats sksl_gpu (inhibits
some inlining!), and if things go according to plan, we'll be removing
enum support from SkSL entirely soon.

Change-Id: If844bbe5fdae41df7930d5c8ea9b832f9dd1b922
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/419099
Reviewed-by: John Stiles <johnstiles@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
19 files changed