Add curve visualization back, guard eval

Editing curves is still cumbersome, but that's fine.
Visualization is just for feedback (and imgui's path
renderer is a little wonky), but this helps a bit.

Bug: skia:
Change-Id: I3dace6d822d472314513bb1ad72bcea1e8991b77
Reviewed-on: https://skia-review.googlesource.com/c/192828
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
4 files changed