move rgb<->hsl sksl to common spot

We'll want to reuse these for other HSL effects, e.g. matrix filter.
Any preference where they go?  New header?

Change-Id: I64534d9801b8bdecf0ef22b3517f8232de998170
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/370712
Reviewed-by: Brian Osman <brianosman@google.com>
2 files changed