label: Microoptimize width-for-height computation

We told Pango to limit width to mid pixels, and it returned a layout
size of text_width by text_height; text_width can be considerably
smaller than mid. If the layout fits, we know that it fits at
text_width, so set max to that. This lets us skip many iterations in a
typical case.

gtk/gtklabel.c

@@ -1202,7 +1202,7 @@ my_pango_layout_get_width_for_height (PangoLayout *layout,
       else if (text_height > for_height)
         min = mid + 1;
       else
-        max = mid;
+        max = text_width;
     }
 
   return min * PANGO_SCALE;