Add RP support for mix(vec, vec, bvec).

This is labeled as `mix_int` but the first two arguments can be any
type, since the type is irrelevant; we are choosing slots from the
first or second argument, not blending between them.

Change-Id: I9699ee12f0920af7ca69ad34da13c1b9697dca32
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/641518
Auto-Submit: John Stiles <johnstiles@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
Reviewed-by: Arman Uguray <armansito@google.com>
6 files changed