backend/x11: use wlr_swapchain

[emersion]

Depends on #2496

This PR switches away from EGLSurface in the X11 backend. Instead, it uses directly the DRI3 and Present extensions.

TODO:

• Destroy back_buffer in output_destroy
• Call wlr_swapchain_set_buffer_submitted
• Intersect render format with DRI3GetSupportedModifiers

Future improvements:

• Don't create a pixmap on each commit -- cache them (backend/x11: re-use pixmaps #2539)
• Use DRI3Open to get the render node (backend/x11: use DRI3Open to get DRM FD  #2540)
• Take advantage of Present to remove the frame timer (backend/x11: implement a real rendering loop #2545)

References: #1352