Support sample(matrix) with runtime effects

Added sksl_sample_chaining, which draws identically to
fp_sample_chaining, but uses runtime effects that implement each
strategy.

Change-Id: Ib54fbe4fc6d98b4a8e91cf0e3ae6b7e19283ad37
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/299076
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
6 files changed