Pass SkPaint by const-ref to SkParticleDrawable

Makes it clearer that this isn't optional, for drawables that route to
similar canvas calls.

Change-Id: I12142c11676ba6baeea3e2779ec05af601d8beb3
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/238440
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
3 files changed