Introduce square/ramp/triangle/round/smooth shape generators,
and use them to seed the range selector coverage pipeline.
Change-Id: Ib7b94ceecd2ccf66820f4dd2443fdd62e2ac6a1b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/218828
Commit-Queue: Florin Malita <fmalita@chromium.org>
Reviewed-by: Ben Wagner <bungeman@google.com>
