output: Add function to set preferred render format

This is an alternative to !3162 (closed) in implementing #1378 (closed), where the new API is a wlr_output_set_render_format function which allows one to set a single format for the output; as opposed to !3162 (closed) 's wlr_output_set_render_format_preference_order function where one can provide a preference ordered list of formats.

I'm still not sure whether this is better than !3162 (closed) , as this shifts some of the complexity in selecting formats outside of wlroots. Edit: I now prefer this to !3162 (closed) .

For a matching Sway implementation, see swaywm/sway!6475 corresponding to branch https://github.com/mstoeckl/sway/commits/bit-depth-setting ,