wl: Set opaque region if possible also when windowed

Configure an opaque region for the Wayland surface also when not
fullscreened, as long as the web view background color is set to be
opaque. This should slightly improve performance in compositors which
take the opaque region hint into account also for non fullscreen
surfaces.

platform/wayland/cog-platform-wl.c

@@ -1605,6 +1605,8 @@ static const struct wl_buffer_listener dmabuf_buffer_listener = {
 static void
 on_export_wl_egl_image(void *data, struct wpe_fdo_egl_exported_image *image)
 {
+    CogWlPlatform *self = data;
+
     const uint32_t surface_pixel_width = wl_data.current_output->scale * win_data.width;
     const uint32_t surface_pixel_height = wl_data.current_output->scale * win_data.height;
 
@@ -1623,14 +1625,17 @@ on_export_wl_egl_image(void *data, struct wpe_fdo_egl_exported_image *image)
 
     if (wpe_view_data.should_update_opaque_region) {
         wpe_view_data.should_update_opaque_region = false;
-        if (win_data.is_fullscreen) {
-          struct wl_region *region =
-              wl_compositor_create_region (wl_data.compositor);
-          wl_region_add (region, 0, 0, win_data.width, win_data.height);
-          wl_surface_set_opaque_region (win_data.wl_surface, region);
-          wl_region_destroy (region);
+
+        WebKitColor bg_color;
+        webkit_web_view_get_background_color(self->web_view, &bg_color);
+
+        if (win_data.is_fullscreen || bg_color.alpha >= 1.0) {
+            struct wl_region *region = wl_compositor_create_region(wl_data.compositor);
+            wl_region_add(region, 0, 0, win_data.width, win_data.height);
+            wl_surface_set_opaque_region(win_data.wl_surface, region);
+            wl_region_destroy(region);
         } else {
-          wl_surface_set_opaque_region (win_data.wl_surface, NULL);
+            wl_surface_set_opaque_region(win_data.wl_surface, NULL);
         }
     }
 
@@ -2513,7 +2518,7 @@ cog_wl_platform_get_view_backend(CogPlatform *platform, WebKitWebView *related_v
     };
 
     wpe_host_data.exportable =
-        wpe_view_backend_exportable_fdo_egl_create(&exportable_egl_client, NULL, win_data.width, win_data.height);
+        wpe_view_backend_exportable_fdo_egl_create(&exportable_egl_client, platform, win_data.width, win_data.height);
     g_assert(wpe_host_data.exportable);
 
     /* init WPE view backend */