Switch from freetype-rs to rusttype

[Boscop]

Like this but for glium_graphics:
PistonDevelopers/gfx_graphics#276

I want to do this (seems like the changes can be pretty much mirrored), but thought I'd better post here in case anyone is already working on this or already did it..

Relevant files:
glium_graphics glyph
gfx_graphics glyph
changes

[bvssvni]

This library needs upgrade to latest piston-graphics. Opened #111. Might need to do this first as there were some changes to the CharacterCache trait.

[bvssvni]

This is ready to be done now.

[gaborigloi]

@Boscop This might also be helpful: https://github.com/dylanede/rusttype/blob/master/examples/gpu_cache.rs#L108 - the GPU glyph cache example of rusttype uses glium already :).
(It seems that gfx_graphics is using its own glyph cache, not the one provided by rusttype.)