Add tests for SkSGMerge

These would have caught the issues with convexity that were in [1]
and fixed in [2]

Also, SkSGPath can be final, so I made it such.

[1] https://skia-review.googlesource.com/c/skia/+/1063416
[2] https://skia-review.googlesource.com/c/skia/+/1065916

Change-Id: I1d2c20b65426072b3441dc87fed29b4c01f8a826
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1065398
Commit-Queue: Kaylee Lubick <kjlubick@google.com>
Reviewed-by: Florin Malita <fmalita@google.com>
2 files changed