backend/drm: fix disappeared output indices
backend/drm: fix disappeared output indices

This commit changes `scan_drm_connectors` to add new outputs to the end of the
list. That way, it's easier to understand what's going on with indices.

When we need to destroy outputs, we now walk the list in reverse order. This
ensures indices remain correct while iterating and removing items from the
list.

We now also make outputs without a CRTC disappear (those are in
WLR_DRM_CONN_NEEDS_MODESET state).

swaywm/wlroots,swaywm/wlroots,SirCmpwn/wlroots,ascent12/wlroots,ascent12/wlroots