wayland: Add support for preferred_buffer_scale/transform

Which got introduced in wl_compositor version 6.

Note that if the surface is visible on multiple monitors with different transforms, we pick the transform of the monitor which we choose for the scale as well. This doesn't really matter at the moment, as the transform is only really relevant for direct-scanout - which we currently only support for fullscreen clients.

Once we support direct-scanout for partially visible clients we'll likely want to introduce a more sophisticated algorithm.