utils: Shuffle and cleanup some paintable code

- It's only used in GcalShellSearchProvider so move it there.
- GdkTexture implements GIcon so no need to convert to a pixbuf.

src/core/gcal-shell-search-provider.c

@@ -70,6 +70,39 @@ static GParamSpec* properties[N_PROPS] = { NULL, };
  * Auxiliary methods
  */
 
+GdkTexture*
+paintable_to_texture (GdkPaintable *paintable)
+{
+  g_autoptr (GtkSnapshot) snapshot = NULL;
+  g_autoptr (GskRenderNode) node = NULL;
+  g_autoptr (GskRenderer) renderer = NULL;
+  g_autoptr (GdkTexture) texture = NULL;
+  g_autoptr (GError) error = NULL;
+  graphene_rect_t viewport;
+
+  snapshot = gtk_snapshot_new ();
+  gdk_paintable_snapshot (paintable, snapshot,
+                          gdk_paintable_get_intrinsic_width (paintable),
+                          gdk_paintable_get_intrinsic_height (paintable));
+  node = gtk_snapshot_free_to_node (g_steal_pointer (&snapshot));
+
+  renderer = gsk_cairo_renderer_new ();
+  gsk_renderer_realize (renderer, NULL, &error);
+  if (error)
+    {
+      g_warning ("Couldn't realize Cairo renderer: %s", error->message);
+      return NULL;
+    }
+
+  viewport = GRAPHENE_RECT_INIT (0, 0,
+                                 gdk_paintable_get_intrinsic_width (paintable),
+                                 gdk_paintable_get_intrinsic_height (paintable));
+  texture = gsk_renderer_render_texture (renderer, node, &viewport);
+  gsk_renderer_unrealize (renderer);
+
+  return g_steal_pointer (&texture);
+}
+
 static gint
 sort_event_data (GcalEvent *a,
                  GcalEvent *b,
@@ -221,8 +254,8 @@ get_result_metas_cb (GcalShellSearchProvider  *self,
   for (i = 0; i < g_strv_length (results); i++)
     {
       g_autoptr (GdkPaintable) paintable = NULL;
+      g_autoptr (GdkTexture) texture = NULL;
       g_autoptr (GVariant) icon_variant = NULL;
-      g_autoptr (GdkPixbuf) gicon = NULL;
 
       uuid = results[i];
       event = g_hash_table_lookup (self->events, uuid);
@@ -232,8 +265,8 @@ get_result_metas_cb (GcalShellSearchProvider  *self,
       g_variant_builder_add (&builder, "{sv}", "name", g_variant_new_string (gcal_event_get_summary (event)));
 
       paintable = get_circle_paintable_from_color (gcal_event_get_color (event), 96);
-      gicon = paintable_to_pixbuf (paintable);
-      icon_variant = g_icon_serialize (G_ICON (gicon));
+      texture = paintable_to_texture (paintable);
+      icon_variant = g_icon_serialize (G_ICON (texture));
       g_variant_builder_add (&builder, "{sv}", "icon", icon_variant);
 
       local_datetime = g_date_time_to_local (gcal_event_get_date_start (event));