[sksl] Introduce the sampleGrad and sampleLod built-ins

Added sampleGrad and sampleLod overloads that work with sampler2D.

Bug: skia:13780, skia:13786
Change-Id: I7026324795380ecccbc9f17bd6deffcedeb25c87
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/589196
Reviewed-by: John Stiles <johnstiles@google.com>
Commit-Queue: Arman Uguray <armansito@google.com>
27 files changed