Add $es3 modifier to SkSL.

This modifier is currently allowed on built-in functions only.

The presence of this modifier will be used to indicate intrinsics which
are ES3-specific (and therefore, not allowed in user code under typical
circumstances).

Change-Id: Ice6be8d9d1b2bf0c8f07f2a89f335bb2f90f6681
Bug: skia:12202
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/439057
Commit-Queue: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
9 files changed