Update some ops to create their buffers in onPrepare.

Need for a follow on change, we need all ops to do all their prep work
which includes creating needed buffers before we start calling
execute on ops. This change updates the few ops that were creating their
buffers in onExecute.

Change-Id: I0f43b0cc35359e1e7ccfb3b159e79bdfe288dcb1
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/239436
Reviewed-by: Chris Dalton <csmartdalton@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>
5 files changed