Wrap page index packing in helper methods.

Also simplifies the shaders if we know there's only one atlas texture.

Change-Id: I81063d423fd56cf91caee83ba9eae9d988aa249b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/266566
Commit-Queue: Jim Van Verth <jvanverth@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
8 files changed