Begin fixing GrShaderVar.

Remove as many setters as possible, make the constructors less
ambiguous, make it movable, remove USE_UNIFORM_FLOAT_ARRAYS.

Change-Id: I71397d04b5b5d6deb792d77cb98d629d42785f06
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/279218
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>
14 files changed