Extract SkFindCubicMidTangent from SkChopCubicAtMidTangent

Also does the same for the quadratic variants.

Bug: skia:10419
Change-Id: I4a0e46a0d76d16dcf452f39c7e2552975ec46ed6
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/318783
Commit-Queue: Chris Dalton <csmartdalton@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
2 files changed