ext_image_copy_capture_session_v1 should have an event to communicate the "native" format

I always send all formats that I can render to. The client is unable to know which one of those is the preferred or native format for the buffer contents. If the client unconditionally chooses xrgb8888, then it might lose data if the source buffer uses argb16161616f. On the other hand, if the client unconditionally chooses a high-depth format, it will waste resources if the source buffer is low depth.