Add a second stencil attachment to GrRenderTarget for dmsaa

As we move toward dmsaa, we need stencil attachments on the single and
multisample attachments both. This is only a temporary solution until
the new surface world is finished.

Bug: skia:11396
Change-Id: I48928343e1fc9fd2e00362a534be9eb3ade92656
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/399838
Commit-Queue: Chris Dalton <csmartdalton@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
40 files changed