Add GrRenderTask and GrOp prePrepare framework

We want to be able to pull more work forward when creating DDLs. The prePrepare entry points will allow us to perform this CPU-side preprocessing.

Change-Id: I2c0c7978dbf7d7c335425ea81ab2323cb9dcdbc3
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/247048
Commit-Queue: Robert Phillips <robertphillips@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>