Content Pipeline fixes for MacOS and Linux

There are a whole bunch of changes there. 
Including the new font generation stuff. 

Because SharpFont requires a full path to the font, I put in a requirement that the font files (.ttf etc) need to be in the same directory as the .spritefont file. You don't need to install the font, however this does change things for windows users. We'll probably need to put in some more fallbacks to system for the various platforms .

The submodule changes bring in the new binaries for nvtt, pvrtc and freetype for mac and in some cases Linux.