Add scaling for vfnts and scale multipler to gr_string()

[MjnMixael]

Adds two features for string handling:

First it adds scaling ability to Volition fonts which brings them into parity with TTF fonts. In doing so it made sense to move the font scale option out of the TTF font cpp file and into the general font cpp file. Notably, allowing players to scale fonts is currently opt-in. Now that all font types can scale, there's possibly an argument to make them opt-out. I could go either way, personally.

Second it adds a scale multiplier to gr_string(). This will be used for the HUD Config upgrade and it basically allows a gr_string() call to multiply the user's font scale up or down by a percentage. Think of it like a C++ version of CSS's em font size capability. This did change the signature of gr_string() because I felt scaleMultiplier had more utility than the length parameter and there were only a handful of places where length was used so it was fairly trivial to update them.