app: make the alpha channel work on canvas overlay widgets

* app/widgets/gimpoverlaychild.c: when creating offscreen windows, try
  to set an RGBA colormap on the offscreen window's widget and use the
  RGBA colormap to create the window. This has to be done here,
  because it's not possible to get to the right screen *inside* the
  offscreen widget before it's parented, and we need that screen
  before the widget is realized, and the widget can't be parented
  before it's realized or it will get the wrong parent window.
  Everything clear now?

* app/widgets/gimpoverlayframe.c: draw the round corners only if the
  screen has an RGBA colormap.

app/widgets/gimpoverlaychild.c

@@ -129,6 +129,8 @@ gimp_overlay_child_realize (GimpOverlayBox   *box,
 {
   GtkWidget     *widget;
   GdkDisplay    *display;
+  GdkScreen     *screen;
+  GdkColormap   *colormap;
   GtkAllocation  child_allocation;
   GdkWindowAttr  attributes;
   gint           attributes_mask;
@@ -139,6 +141,11 @@ gimp_overlay_child_realize (GimpOverlayBox   *box,
   widget = GTK_WIDGET (box);
 
   display = gtk_widget_get_display (widget);
+  screen  = gtk_widget_get_screen (widget);
+
+  colormap = gdk_screen_get_rgba_colormap (screen);
+  if (colormap)
+    gtk_widget_set_colormap (child->widget, colormap);
 
   gtk_widget_get_allocation (child->widget, &child_allocation);
 
@@ -157,9 +164,9 @@ gimp_overlay_child_realize (GimpOverlayBox   *box,
   attributes.y           = child_allocation.y;
   attributes.window_type = GDK_WINDOW_OFFSCREEN;
   attributes.wclass      = GDK_INPUT_OUTPUT;
-  attributes.visual      = gtk_widget_get_visual (widget);
+  attributes.visual      = gtk_widget_get_visual (child->widget);
-  attributes.colormap    = gtk_widget_get_colormap (widget);
+  attributes.colormap    = gtk_widget_get_colormap (child->widget);
-  attributes.event_mask  = gtk_widget_get_events (widget);
+  attributes.event_mask  = gtk_widget_get_events (child->widget) | GDK_EXPOSURE_MASK;
   attributes.cursor      = gdk_cursor_new_for_display (display, GDK_LEFT_PTR);
 
   attributes_mask = (GDK_WA_X        |
@@ -345,11 +352,13 @@ gimp_overlay_child_expose (GimpOverlayBox   *box,
     }
   else if (event->window == child->window)
     {
-      gtk_paint_flat_box (gtk_widget_get_style (widget),
+#if 0
+      gtk_paint_flat_box (gtk_widget_get_style (child->widget),
                           event->window,
                           GTK_STATE_NORMAL, GTK_SHADOW_NONE,
                           &event->area, widget, NULL,
                           0, 0, -1, -1);
+#endif
 
       gtk_container_propagate_expose (GTK_CONTAINER (widget),
                                       child->widget,

app/widgets/gimpoverlayframe.c

@@ -53,19 +53,7 @@ gimp_overlay_frame_class_init (GimpOverlayFrameClass *klass)
 static void
 gimp_overlay_frame_init (GimpOverlayFrame *frame)
 {
-  GtkWidget   *widget = GTK_WIDGET (frame);
+  gtk_widget_set_app_paintable (GTK_WIDGET (frame), TRUE);
-
-#if 0 /* crashes badly beause gtk+ doesn't support offscreen windows
-       * with colormap != parent_colormap yet
-       */
-  GdkScreen   *screen = gtk_widget_get_screen (widget);
-  GdkColormap *rgba   = gdk_screen_get_rgba_colormap (screen);
-
-  if (rgba)
-    gtk_widget_set_colormap (widget, rgba);
-#endif
-
-  gtk_widget_set_app_paintable (widget, TRUE);
 }
 
 static void
@@ -124,6 +112,23 @@ gimp_overlay_frame_expose (GtkWidget      *widget,
 {
   cairo_t  *cr    = gdk_cairo_create (gtk_widget_get_window (widget));
   GtkStyle *style = gtk_widget_get_style (widget);
+  gboolean  rgba;
+
+  rgba = gdk_screen_get_rgba_colormap (gtk_widget_get_screen (widget)) != NULL;
+
+  if (rgba)
+    {
+      cairo_set_operator (cr, CAIRO_OPERATOR_CLEAR);
+      gdk_cairo_region (cr, eevent->region);
+      cairo_clip_preserve (cr);
+      cairo_fill (cr);
+    }
+
+  cairo_set_operator (cr, CAIRO_OPERATOR_OVER);
+  gdk_cairo_set_source_color (cr, &style->bg[GTK_STATE_NORMAL]);
+
+  if (rgba)
+    {
       GtkAllocation  allocation;
       gint           border_width;
 
@@ -131,14 +136,6 @@ gimp_overlay_frame_expose (GtkWidget      *widget,
 
       border_width = gtk_container_get_border_width (GTK_CONTAINER (widget));
 
-  cairo_set_operator (cr, CAIRO_OPERATOR_CLEAR);
-  gdk_cairo_region (cr, eevent->region);
-  cairo_clip_preserve (cr);
-  cairo_fill (cr);
-
-  cairo_set_operator (cr, CAIRO_OPERATOR_OVER);
-  gdk_cairo_set_source_color (cr, &style->bg[GTK_STATE_NORMAL]);
-
 #define DEG_TO_RAD(deg) ((deg) * (G_PI / 180.0))
 
       cairo_arc (cr,
@@ -178,6 +175,11 @@ gimp_overlay_frame_expose (GtkWidget      *widget,
                  DEG_TO_RAD (90),
                  DEG_TO_RAD (180));
       cairo_close_path (cr);
+    }
+  else
+    {
+      gdk_cairo_region (cr, eevent->region);
+    }
 
   cairo_fill (cr);