Metal: Use MipmapNativeLevel class to store native mip lvl This is to avoid wrong native level in Metal, for example, not taking into account the OpenGL base level. Bug: angleproject:2634 Change-Id: I3a7a3ac41bdbd91a47755bb4ca11bd579c182e04 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2433326 Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org>