Replace DSL's Sample() with DSLGlobalVar::eval()

Bug: skia:12302
Change-Id: I7ff7bae388c5991f2c23c8945355fea55c42095a
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/447436
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
8 files changed