Fix vertical font padding in SharpFontImporter

This fixes the vertical font padding in the SharpFontImporter.  Currently fonts imported in the MonoGame content pipeline look like this when rendered:

![fontbroken](https://f.cloud.github.com/assets/2063515/1771050/e1b8c088-67a7-11e3-8560-0538db765e65.PNG)

This change ensures that fonts are rendered correctly; after this change it looks like this:

![fontfixed](https://f.cloud.github.com/assets/2063515/1771052/f2a0a122-67a7-11e3-8804-b18731086f5c.PNG)