TEMPORARY: track stencil clip state in GrRenderTargetOpList

Tracks the stencil clip state in GrRenderTargetOpList instead of
GrStencilAttachment. This is a temporary move to unblock MDB, after
which point we will be able to overhaul clipping.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2468743002

Review-Url: https://codereview.chromium.org/2468743002
5 files changed