Make GrOp::onPrePrepare be pure virtual

This makes onPrePrepare match onPrepare & onExecute. Most of the new method bodies will-have-to/should be filled in anyway.

Change-Id: Ifc897feaeb2d8fe6eec3ac3a8e91f99393ad6758
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/277542
Commit-Queue: Robert Phillips <robertphillips@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
20 files changed