protocol: deprecate non-current wl_output.mode
The current wl_output.mode event has several issues when used to advertise modes that aren't current:
- It's not possible to remove some modes. This is an issue for virtual outputs and when the kernel prunes some modes because of link limitations.
- wl_output.mode fails to carry metadata such as aspect ratio, which results in duplicated or missing modes.
- It's not clear, given the current set of protocols, how non-current modes are useful to clients. Xwayland ignores non-current modes.
GNOME and wlroots already only advertise the current mode because of these issues.
If a protocol needs the clients to know about all available modes, it should advertise these modes itself instead of relying on wl_output.mode.