Move GrDrawOp pipeline/clip processing to GrRenderTargetContext

Move GrDrawOp pipeline/clip processing to GrRenderTargetContext

This is currently done in GrOpList. However, it can trigger resource creation, which in turn can trigger a flush. In the future flushing may destroy the op list.

Change-Id: I21cb1e10060bf31c95431c0511fcfff637cd6498
Reviewed-on: https://skia-review.googlesource.com/9304
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>