step 5: slides_scene amb wipe suau per easing (substituix doSlides)

source/scenes/surface_handle.cpp

@@ -33,4 +33,10 @@ void SurfaceHandle::adopt(JD8_Surface raw) {
     surface_ = raw;
 }
 
+JD8_Surface SurfaceHandle::release() {
+    JD8_Surface r = surface_;
+    surface_ = nullptr;
+    return r;
+}
+
 }  // namespace scenes