color toning: properly draw hue and strength gradients for the highlights and shadows sliders

Fixes #4209

rtgui/thresholdselector.cc

@@ -284,6 +284,9 @@ void ThresholdSelector::updateBackBuffer()
     cr->set_line_cap(Cairo::LINE_CAP_BUTT);
 
     if (is_sensitive() && coloredBar.canGetColors()) {
+        if (updatePolicy == RTUP_DYNAMIC) {
+            coloredBar.setDirty(true);
+        }
         // this will eventually create/update the off-screen Surface for the gradient area only !
         coloredBar.setDrawRectangle(hb + hwslider, int(float(h) * 1.5f / 7.f + 0.5f), iw + 1, int(float(h) * 4.f / 7.f + 0.5f));
         // that we're displaying here