GFX/DISPLAY: Restore static on gfx_display_draw_bg coords

The struct's address is stored in the caller's draw struct and
read after gfx_display_draw_bg returns, so the storage must
outlive the call. Making it a plain stack local produced a
dangling pointer, breaking wallpaper and pipeline background
rendering in xmb_frame.

The original thread-safety concern from 0abe6dc stands, but the
correct fix is for callers to own the video_coords struct and
pass it in — a larger refactor. Restoring static as an interim
fix.

gfx/gfx_display.c

@@ -461,7 +461,7 @@ void gfx_display_draw_bg(
       void *userdata, bool add_opacity_to_wallpaper,
       float override_opacity)
 {
-   struct video_coords coords;
+   static struct video_coords coords;
    const float           *new_vertex = NULL;
    const float        *new_tex_coord = NULL;
    gfx_display_ctx_driver_t *dispctx = p_disp->dispctx;