Track if we need stencil on GrRenderTargetContext

There are a few places that have been checking whether the *proxy*
needs stencil, in order to determine if the current render target
context needs stencil. This is problematic since a render target
context can not require stencil itself, but wrap an existing proxy
that already has stencil.

Bug: skia:
Change-Id: I2719dd3a9df15fef3d64f991cda4fadea23266bb
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/223970
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Chris Dalton <csmartdalton@google.com>
6 files changed