Remove unnecessary casting from SkSL in GMs

Converts examples to use more idiomatic "user" SkSL,
without extra casts, and taking advantage of swizzle.

Change-Id: I4ad4e7b6563b4f09402855cb125546b015622ced
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/317388
Reviewed-by: Mike Klein <mtklein@google.com>
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
4 files changed