Debug groups and signposts on render passes #205

Reading https://developer.apple.com/documentation/metal/frame_capture_debugging_tools/enhancing_frame_capture_by_using_labels?language=objc gets me confused. So the pushDebugGroup and friends are only defined on MTLCommandEncoder, and yet their own example shows how to use it on MTLRenderPassEncoder.

If we can call it there, metal-rs should provide that API.