st-drawing-area: Avoid unnecessary repaints

StDrawingArea uses the member variable needs_repaint to keep track
of whether it needs repainting. The variable is set to TRUE correctly,
e.g. on allocation or style changes - alas, it is never set to FALSE,
resulting in the area being repainted continuously.

https://bugzilla.gnome.org/show_bug.cgi?id=632197

src/st/st-drawing-area.c

@@ -115,6 +115,7 @@ st_drawing_area_paint (ClutterActor *self)
           surface = cairo_image_surface_create (CAIRO_FORMAT_ARGB32, width, height);
           priv->context = cairo_create (surface);
           priv->in_repaint = TRUE;
+          priv->needs_repaint = FALSE;
 
           g_signal_emit ((GObject*)area, st_drawing_area_signals[REPAINT], 0);