Implement exp2 intrinsic in SkRP.

Fortunately, we had all the moving parts ready to go; it was just a
matter of connecting them together.

Change-Id: I3a7b3ae71c8405de8497f0cd5f88d01c8b633865
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/659876
Commit-Queue: Brian Osman <brianosman@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
5 files changed