Metal: Fix discardable MSAA when used with stencil.

The RenderCommandEncoder created for LoadMSAAFromResolve needs to be
able to merge with the encoder that follows it -- otherwise, the load
will do nothing. Hence if the following encoder has stencil, we need to
add stencil to the LoadFromMSAAResolve encoder.

Bug: skia:12086
Change-Id: Idf1249e386e1a24c4f18477e7ff67910bc21aa3e
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/446716
Commit-Queue: Jim Van Verth <jvanverth@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
8 files changed