Font fixes and changes

* Removed OS X specific Derma font. This now uses Tahoma like Windows.
* Added Linux specific Derma font. Linux distros are more likely to have the close-looking DejaVu Sans than Tahoma. _This MAY fix the italic font some people had in Derma but will NOT fix surface.CreateFont using an italic font as a default if the font cannot be found. I was unable to reproduce this so I cannot confirm whether it even fixes Derma._
* Added Windows and Linux specific fonts to the Workshop download VGUI. Before this only had Helvetica which these systems may not have.