SkSL: Simplify layout(key)

We had support for layout(key=identity), which added a bit to the key
depending on whether or not a matrix variable was identity. That feature
was unused - now 'key' is just a flag, rather than a separate enum.

Change-Id: I691a0b3683610f55e5d020b9cd6d9e414690bee6
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/374416
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
9 files changed