x11: Add function to know whether a client X11 window has an alpha channel

Since the windows created by the frames client will have a RGBA visual, we
no longer can perform simple tests about whether the window is opaque. For
that, we will need to additionally know whether the client-side window has
a visual with an alpha channel.

Reviewed-by: Michel Dänzer <mdaenzer@redhat.com>
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2758>

src/x11/window-x11.c

@@ -4331,3 +4331,30 @@ meta_window_x11_check_update_resize (MetaWindow *window)
                              window->display->grab_latest_motion_x,
                              window->display->grab_latest_motion_y);
 }
+
+gboolean
+meta_window_x11_has_alpha_channel (MetaWindow *window)
+{
+  MetaX11Display *x11_display = window->display->x11_display;
+  int n_xvisuals;
+  gboolean has_alpha;
+  XVisualInfo *xvisual_info;
+  XVisualInfo template = {
+    .visualid = XVisualIDFromVisual (window->xvisual),
+  };
+
+  xvisual_info = XGetVisualInfo (meta_x11_display_get_xdisplay (x11_display),
+                                 VisualIDMask,
+                                 &template,
+                                 &n_xvisuals);
+  if (!xvisual_info)
+    return FALSE;
+
+  has_alpha = (xvisual_info->depth >
+               __builtin_popcount (xvisual_info->red_mask |
+                                   xvisual_info->green_mask |
+                                   xvisual_info->blue_mask));
+  XFree (xvisual_info);
+
+  return has_alpha;
+}

src/x11/window-x11.h

@@ -107,4 +107,6 @@ gboolean meta_window_x11_is_awaiting_sync_response (MetaWindow *window);
 
 void meta_window_x11_check_update_resize (MetaWindow *window);
 
+gboolean meta_window_x11_has_alpha_channel (MetaWindow *window);
+
 #endif