Fix corner rounding in uiwidgets.ResizeableFrame

This improves the quality of rounded corners in, e.g., list and variable watchers.

Before/after:
![screen shot 2014-07-08 at 14 08 52](https://cloud.githubusercontent.com/assets/1578238/3514313/15615900-06cb-11e4-8852-73cc6e7e984f.png) ![screen shot 2014-07-08 at 14 08 58](https://cloud.githubusercontent.com/assets/1578238/3514315/1829cfe6-06cb-11e4-97d5-354a9b3b2767.png)