Clip to avoid glitches when drawing some non-monospaced graphical chars

Clip rendering area to avoid characters being drawn bigger than expected (bug with supposed-to-be-monsopaced font)

Without clipping (notice the white pixel just over the mouse, and the double right columns) :
![yast](https://f.cloud.github.com/assets/3908084/1186489/73a6e9f6-2302-11e3-93cd-3dbb952d266e.png)

With clipping (no more corruption, but some graphical chars are cut; this is inevitable) :
![yast_clipped](https://f.cloud.github.com/assets/3908084/1186490/7513e686-2302-11e3-9711-b7540bba53ba.png)