backend/drm: Implement support for renderer loss recovery

backend/drm: Implement support for renderer loss recovery

This implementation is nearly identical to Sway's, except that
it also reloads the configuration, to spur on reloading the
server-side decorations.

v2: Fix style.
v3: Add a reset to the magnifier.
v4: Oops, restructure reset handler a bit.
v5: Commit the magnifier reset immediately, before freeing the
    lost allocator and renderer.
v6: Also check for failed render pass, which may return NULL.
v7: Add a second NULL test, just in case.