Use glyph metrics for spacing.

Currently the characters are spaced using their bounding width and fixed size. That can look very bad with some fonts. Using appropriate metrics given by FreeType library looks much better.

This changes the GBP file format by adding two fields to the TTFGlyph structure. Is such change acceptable? Is anything else required for it?

Illustration of the change with Chopin Script:
http://www.dafont.com/chopin-script.font
![font-spacing-comparison](https://cloud.githubusercontent.com/assets/1572230/3756352/744546e8-182d-11e4-8976-514bdbbefad9.png)