Use wlroots textures for client buffer imports

Instead of maintaining our own code to load and update SHM and dmabuf buffers of clients we leverage wlroots from now on for this. Since our internal non-accelerated buffers are basically plain pixel buffers like SHM buffers are, we can leverage wlroots for these too.

This not only allows us to remove a lot of code but also directly increases our feature set as now we can advertise all dmabuf buffer formats that wlroots supports while before we had to limit it to few single-plane formats.