Fix frame buffer: clear to transparent so children show through

The frame buffer was cleared to opaque black, covering the menu
content child buffer. Now cleared to alpha=0 (transparent).

src/backend/wayland/wl_backend.c

@@ -3700,7 +3700,7 @@ wl_frame_paint(struct WFrameWindow *fwin)
     int total_h = fb->height;
 
     /* Clear the buffer to transparent black. */
-    pixman_color_t clear = {0, 0, 0, 0xFFFF};
+    pixman_color_t clear = {0, 0, 0, 0};  /* transparent */
     pixman_image_fill_rectangles(PIXMAN_OP_SRC, fb->image, &clear,
         1, &(pixman_rectangle16_t){0, 0, total_w, total_h});