Update gradient colorizers to take coords instead of colors.

Gradient layout FPs were passing a position to their colorizers via
sk_InColor.x. This worked, but it makes more sense to pass coordinates
to children by sampling the child at explicit coordinates.

Change-Id: I60520f2e54e04bc9c4d8562e6b2256b95e0de5b4
Bug: skia:10548
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/307217
Reviewed-by: Brian Salomon <bsalomon@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
20 files changed