rewrite SkMasks::CreateMasks to work in bytesPerPixel

This ought to make it a bit more clear that undefined
shifts (like 1<<31, 1<<32, 1<<33) are not possible,
only 1<<8, 1<<16, 1<<24.

Change-Id: Ia358f9204e5956ba6c23603c5961af86a991b659
Reviewed-on: https://skia-review.googlesource.com/c/192030
Reviewed-by: Leon Scroggins <scroggo@google.com>
Reviewed-by: Greg Kaiser <gkaiser@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>