Fix clamp lowbits extraction when combined with non-clamp

R=reed@google.com

Change-Id: I1dc17c827077e968d4dbcd0f9ea8da8e1732b253
Reviewed-on: https://skia-review.googlesource.com/6231
Commit-Queue: Florin Malita <fmalita@google.com>
Reviewed-by: Mike Reed <reed@google.com>
2 files changed