[graphite] Create graphite::Image_Base

First steps to adding graphite::Image_YUVA. Splits off any shared
functionality.

Bug: b/238756380
Change-Id: I7235ea553a936f691f6b01e13f9d29383dc1b358
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/630698
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
5 files changed