Use an ARGB visual if one is available (FS#829)

With this commit, awesome prefers ARGB visuals over the screen's default visual.
This means that all our (visible) windows now can get an alpha channel that a
compositing manager can use for producing transparent windows.

The reason why this is done is to fix a bug. We are reparenting clients into
other windows. If one of these client window uses an ARGB visual, its
transparency would have no effect.

Signed-off-by: Uli Schlachter <psychon@znc.in>

awesome.c

@@ -157,7 +157,7 @@ scan(void)
 }
 
 static xcb_visualtype_t *
-screen_default_visual(xcb_screen_t *s)
+a_default_visual(xcb_screen_t *s)
 {
     xcb_depth_iterator_t depth_iter = xcb_screen_allowed_depths_iterator(s);
 
@@ -171,6 +171,36 @@ screen_default_visual(xcb_screen_t *s)
     return NULL;
 }
 
+static xcb_visualtype_t *
+a_argb_visual(xcb_screen_t *s)
+{
+    xcb_depth_iterator_t depth_iter = xcb_screen_allowed_depths_iterator(s);
+
+    if(depth_iter.data)
+        for(; depth_iter.rem; xcb_depth_next (&depth_iter))
+            if(depth_iter.data->depth == 32)
+                for(xcb_visualtype_iterator_t visual_iter = xcb_depth_visuals_iterator(depth_iter.data);
+                    visual_iter.rem; xcb_visualtype_next (&visual_iter))
+                    return visual_iter.data;
+
+    return NULL;
+}
+
+static uint8_t
+a_visual_depth(xcb_screen_t *s, xcb_visualid_t vis)
+{
+    xcb_depth_iterator_t depth_iter = xcb_screen_allowed_depths_iterator(s);
+
+    if(depth_iter.data)
+        for(; depth_iter.rem; xcb_depth_next (&depth_iter))
+            for(xcb_visualtype_iterator_t visual_iter = xcb_depth_visuals_iterator(depth_iter.data);
+                visual_iter.rem; xcb_visualtype_next (&visual_iter))
+                if(vis == visual_iter.data->visual_id)
+                    return depth_iter.data->depth;
+
+    fatal("Could not find a visual's depth");
+}
+
 static void
 a_refresh_cb(EV_P_ ev_prepare *w, int revents)
 {
@@ -374,9 +404,19 @@ main(int argc, char **argv)
         fatal("cannot open display");
 
     globalconf.screen = xcb_aux_get_screen(globalconf.connection, globalconf.default_screen);
-    globalconf.visual = screen_default_visual(globalconf.screen);
+    globalconf.visual = a_argb_visual(globalconf.screen);
-    globalconf.default_depth = globalconf.screen->root_depth;
+    if(!globalconf.visual)
+        globalconf.visual = a_default_visual(globalconf.screen);
+    globalconf.default_depth = a_visual_depth(globalconf.screen, globalconf.visual->visual_id);
     globalconf.default_cmap = globalconf.screen->default_colormap;
+    if(globalconf.default_depth != globalconf.screen->root_depth)
+    {
+        // We need our own color map if we aren't using the default depth
+        globalconf.default_cmap = xcb_generate_id(globalconf.connection);
+        xcb_create_colormap(globalconf.connection, XCB_COLORMAP_ALLOC_NONE,
+                globalconf.default_cmap, globalconf.screen->root,
+                globalconf.visual->visual_id);
+    }
 
     /* Prefetch all the extensions we might need */
     xcb_prefetch_extension_data(globalconf.connection, &xcb_big_requests_id);