Improve multi-GPU buffer sharing
On multi-GPU setups we render everything on one primary GPU and then we copy buffers around to display them on secondary GPUs. This is done by exporting a DMA-BUF on the primary GPU, importing it on the secondary GPU, and rendering it on the secondary GPU:
Instead we should use direct scan-out to skip rendering on the secondary GPU. This could be the groundwork for https://github.com/swaywm/wlroots/issues/1270.