Use exp2 instead of ldexp/shifts in the middle out cubic shader

Change-Id: I6cb55aa6df3b5477adf036eea34079ff2ec33488
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/313150
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Chris Dalton <csmartdalton@google.com>
1 file changed