Add functionality for loading specific font faces from .ttc files

[AlexHarker]

This pull request adds two font loading functions that allow loading faces from .ttc files (on disk or in memory) given an index for the face. This involves changes through into fontstash.h to update font loading to support this. This should support this functionality for either stb_true_type or freetype.

There are also some minor cosmetic fixes in the other two commits.