Metal: more fixes for stencil and discardable MSAA

* When loading the MSAA texture from resolve, we set up the render
command encoder to be compatible with the next op. So we should just use
that command encoder rather than create one. This mainly helps with
clears as detecting compatibility in that case is difficult.

Bug: skia:12086
Change-Id: I859761ff970520c36629f1b8054f73e2fd29eb67
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/447189
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
5 files changed