wl_backend: fix standalone VT switching and relative pointer motion

- Store wlr_session from wlr_backend_autocreate.
- Skip frame commit when session is inactive (VT switched away).
- Wire ptr->events.motion (relative) for libinput devices so cursor
  moves in standalone DRM/KMS mode.

src/backend/wayland/wl_backend.c

@@ -116,6 +116,7 @@
 #include <wayland-server-core.h>
 #include <wlr/backend.h>
 #include <wlr/backend/headless.h>
+#include <wlr/backend/session.h>
 #include <wlr/render/wlr_renderer.h>
 #include <wlr/interfaces/wlr_output.h>
 #include <wlr/types/wlr_compositor.h>
@@ -386,6 +387,7 @@ struct wl_compositor_state {
     int                             is_nested;           /* 1 if using wlr_wl_backend */
     struct wlr_backend             *headless_inner;      /* headless backend (always) */
     struct wlr_backend             *wl_backend_inner;    /* wl backend (nested only) */
+    struct wlr_session             *session;             /* DRM session (standalone only) */
     struct wlr_allocator           *allocator;
     struct wlr_scene               *scene;
     struct wlr_scene_output        *scene_output;
@@ -1810,10 +1812,10 @@ static void
 handle_output_frame(struct wl_listener *listener, void *data)
 {
     (void)listener; (void)data;
-    /* Skip rendering while the startup pump thread is running — wlr_scene_output_commit
-     * involves EGL/DRM operations bound to the main thread's GL context. */
     if (wl_state.startup_pump_running)
         return;
+    if (wl_state.session && !wl_state.session->active)
+        return;
     if (!wl_state.scene_output)
         return;
     if (wl_state.allocator) {
@@ -2092,6 +2094,7 @@ wl_display_open(const char *name)
         struct wlr_session *session = NULL;
         wl_state.backend = wlr_backend_autocreate(
             wl_display_get_event_loop(wl_state.display), &session);
+        wl_state.session = session;
         if (!wl_state.backend) {
             wwarning("wl_backend: wlr_backend_autocreate failed; falling back to headless");
             wl_state.backend = wl_state.headless_inner;