store the page bits in U only

Store the 2 page index bits in bits 14 and 15 of U. This allows
several simplifications.

Change-Id: Ib3ce98f0f5a430849acc0e6f6f3ecfabdb3f038d
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/313896
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Herb Derby <herb@google.com>
2 files changed