[canvaskit] Replace Postan value array with Float32Array.

Change-Id: Ie0e8a9271ef1b68551473fb9a1bcc9442f063efe
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/362477
Reviewed-by: Nathaniel Nifong <nifong@google.com>
8 files changed