Revert removal of automatic drm reset on VT switch

...on the basis that it reduces user-space breakage with respect to
layer-shell clients disappearing on suspend/resume and on VT change.

Revert the following two commits:

- 1edd5e2  "backend/drm: Remove reset from interface"
- 0f255b4  "backend/drm: Remove automatic reset on VT switch"

This it not intended to be reverted on the `master` branch. Instead, the
next wlroots release is anticipated to handle the situation differently,
possibly by splitting some objects and leaving output-related wl_globals
which have been announced to clients. See discussions on [issue-3944].

[issue-3944]: https://gitlab.freedesktop.org/wlroots/wlroots/-/issues/3944#note_3030632

Background:

With [MR-4878] on wlroots-0.19.0, the DRM backend destroys/recreates
outputs on VT switch and in some cases on suspend/resume too. The reason
for this change was that (i) the KMS state is undefined when a VT is
switched away; and (ii) the previous outputs had issues with restoration,
particularly when the output configuration had changed whilst switched
away. This change causes two issues for users:

- Some layer-shell clients do not re-appear on output re-connection, or
  may appear on a different output. Whilst this has always been the case,
  it will now also happen in said situations. It is technically possible
  for layer-shell clients to deal with this more thoughtfully by handling
  the new-output and surface-destroy signals to achieve desired
  behaviours, but such changes take time and meanwhile Desktop Environment
  teams and other users consider this a serious regression.

- Some Gtk clients issue critical warnings on some compositors as they
  assume that at least one output is always available. This will be fixed
  in `Gtk-3.24.50` and is believed to be a harmless warning,

[MR-4878]: https://gitlab.freedesktop.org/wlroots/wlroots/-/merge_requests/4878

Testing:

1. This reversion has been tested with [conky] and [GlassyClock]. With the
reversion applied, the clients remain visible after VT switching.
2. It was also tested with labwc on tty1; and sway on tty2. No adverse
affects were observed when changing output scales on either compositor.

[conky]: https://github.com/brndnmtthws/conky
[GlassyClock]: https://github.com/tsujan/GlassyClock

backend/drm/drm.c

@@ -1888,6 +1888,108 @@ void scan_drm_leases(struct wlr_drm_backend *drm) {
 	drmFree(list);
 }
 
+static void build_current_connector_state(struct wlr_output_state *state,
+		struct wlr_drm_connector *conn) {
+	bool enabled = conn->status != DRM_MODE_DISCONNECTED && conn->output.enabled;
+
+	wlr_output_state_init(state);
+	wlr_output_state_set_enabled(state, enabled);
+	if (!enabled) {
+		return;
+	}
+
+	if (conn->output.current_mode != NULL) {
+		wlr_output_state_set_mode(state, conn->output.current_mode);
+	} else {
+		wlr_output_state_set_custom_mode(state,
+			conn->output.width, conn->output.height, conn->output.refresh);
+	}
+}
+
+/**
+ * Check whether we need to perform a full reset after a VT switch.
+ *
+ * If any connector or plane has a different CRTC, we need to perform a full
+ * reset to restore our mapping. We couldn't avoid a full reset even if we
+ * used a single KMS atomic commit to apply our state: the kernel rejects
+ * commits which migrate a plane from one CRTC to another without going through
+ * an intermediate state where the plane is disabled.
+ */
+static bool skip_reset_for_restore(struct wlr_drm_backend *drm) {
+	struct wlr_drm_connector *conn;
+	wl_list_for_each(conn, &drm->connectors, link) {
+		drmModeConnector *drm_conn = drmModeGetConnectorCurrent(drm->fd, conn->id);
+		if (drm_conn == NULL) {
+			return false;
+		}
+		struct wlr_drm_crtc *crtc = connector_get_current_crtc(conn, drm_conn);
+		drmModeFreeConnector(drm_conn);
+
+		if (crtc != NULL && conn->crtc != crtc) {
+			return false;
+		}
+	}
+
+	for (size_t i = 0; i < drm->num_planes; i++) {
+		struct wlr_drm_plane *plane = &drm->planes[i];
+
+		drmModePlane *drm_plane = drmModeGetPlane(drm->fd, plane->id);
+		if (drm_plane == NULL) {
+			return false;
+		}
+		uint32_t crtc_id = drm_plane->crtc_id;
+		drmModeFreePlane(drm_plane);
+
+		struct wlr_drm_crtc *crtc = NULL;
+		for (size_t i = 0; i < drm->num_crtcs; i++) {
+			if (drm->crtcs[i].id == crtc_id) {
+				crtc = &drm->crtcs[i];
+				break;
+			}
+		}
+		if (crtc == NULL) {
+			continue;
+		}
+
+		bool ok = false;
+		switch (plane->type) {
+		case DRM_PLANE_TYPE_PRIMARY:
+			ok = crtc->primary == plane;
+			break;
+		case DRM_PLANE_TYPE_CURSOR:
+			ok = crtc->cursor == plane;
+			break;
+		}
+		if (!ok) {
+			return false;
+		}
+	}
+
+	return true;
+}
+
+void restore_drm_device(struct wlr_drm_backend *drm) {
+	// The previous DRM master leaves KMS in an undefined state. We need
+	// to restore our own state, but be careful to avoid invalid
+	// configurations. The connector/CRTC mapping may have changed, so
+	// first disable all CRTCs, then light up the ones we were using
+	// before the VT switch.
+	// TODO: better use the atomic API to improve restoration after a VT switch
+	if (!skip_reset_for_restore(drm) && !drm->iface->reset(drm)) {
+		wlr_log(WLR_ERROR, "Failed to reset state after VT switch");
+	}
+
+	struct wlr_drm_connector *conn;
+	wl_list_for_each(conn, &drm->connectors, link) {
+		struct wlr_output_state state;
+		build_current_connector_state(&state, conn);
+		if (!drm_connector_commit_state(conn, &state, false)) {
+			wlr_drm_conn_log(conn, WLR_ERROR, "Failed to restore state after VT switch");
+		}
+		wlr_output_state_finish(&state);
+	}
+}
+
 bool commit_drm_device(struct wlr_drm_backend *drm,
 		const struct wlr_backend_output_state *output_states, size_t output_states_len,
 		bool test_only) {