Metal: Use render passes for B2T PBO copies. This would make it possible to avoid setting ShaderWrite usage to all textures in future. Bug: chromium:392521084 Change-Id: I25145f3578589c71ee469df08162d3a3f17c8cbe Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/6356129 Reviewed-by: Kenneth Russell <kbr@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Quyen Le <lehoangquyen@chromium.org>