app: GimpRuler: clip position marker drawing to the ruler's allocation

so we don't overdraw when drawing out of expose.

libgimpwidgets/gimpruler.c

@@ -1192,6 +1192,11 @@ gimp_ruler_draw_pos (GimpRuler *ruler)
       gdouble  position;
       gdouble  increment;
 
+      cairo_rectangle (cr,
+                       allocation.x, allocation.y,
+                       allocation.width, allocation.height);
+      cairo_clip (cr);
+
       cairo_translate (cr, allocation.x, allocation.y);
 
       /*  If a backing store exists, restore the ruler  */
@@ -1202,7 +1207,6 @@ gimp_ruler_draw_pos (GimpRuler *ruler)
           cairo_fill (cr);
         }
 
-
       position = gimp_ruler_get_position (ruler);
 
       gimp_ruler_get_range (ruler, &lower, &upper, NULL);