Metal: fix wrong render encoder's serial assignment mtl::RenderCommandEncoder object is created once and reused multiple times. We should re-assign a new serial whenever we reuse it, instead of assigning once in constructor. bug: angleproject:40644888 Change-Id: I18120c91dddd5b279fcec7a9099833e97ec1c37c Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/5746179 Reviewed-by: Geoff Lang <geofflang@chromium.org> Reviewed-by: Kenneth Russell <kbr@chromium.org> Commit-Queue: Quyen Le <lehoangquyen@chromium.org>