[mesh2d demo] Patch controls

Introduce a "manual" mode, which implements Coons patch deformation
using draggable cubic Bezier controls for each corner.

Change-Id: I36169edc59c75914219e05e2cc220404540de864
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/756316
Commit-Queue: Florin Malita <fmalita@chromium.org>
Reviewed-by: Kevin Lubick <kjlubick@google.com>
1 file changed