Allow more sub-pixel position bits

I tracked down all the places where things were assuming 2-bits
of sub-pixel positions. I generalized them all to take a variable
number of bits.

Change-Id: I304061635c838fedc3881aaa256bab943ff6a817
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/253236
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Herb Derby <herb@google.com>
4 files changed