Fixed GWEN TextureBorder typo

The previous code was wrong because _w-_l-_r will return uv x value plus
the width between the left and the right border, while it should be
_w-_r to return the position where the right border starts. This is not
noticeable on the current derma panels, but if you want to make your own
with more features on it, it will be clear that something is wrong.
Merge pull request #772 from Metamist/master

Fixed GWEN TextureBorder mistake