Add convexity tests when adding a (convex) path

Use the same logic in addPath as we (now) have in add[Rect,Oval,RRect]
for deciding that we are 'effectively' empty before the add, and
therefore we can be considered convex afterwards.

Also, this should fix unit-test in flutter.

Thought:

- not sure why we only do the 'replace' step if matrix is I()
- seems like we can still replace ourselves if we have to first
  transform the srcPath
- may investigate that in a separate CL

Change-Id: Ib7bacc6635e65c88045c2ce9edfb6be52474db65
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1035076
Commit-Queue: Florin Malita <fmalita@google.com>
Reviewed-by: Ben Wagner <bungeman@google.com>
Reviewed-by: Florin Malita <fmalita@google.com>
4 files changed