Includes a general explanation of device vs. local, how shaders don't
move with geometry, and how to use canvas transforms. Also shows
localMatrix on an SkShader. Finally, links to that from the SkSL page,
and adds SkSL-specific notes about coordinates.
Bug: skia:11763
Change-Id: I31462aff66ff8514392098b972a10e7c96e4f254
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/393516
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
