With depth testing enabled nearby glyphs can z-fight
and so some pixels might be discarded before fragment shader
even runs, which causes us to see only part of a letter.
As such we always draw with blending enabled and depth testing
disabled so that any overlaps simply causes the char pixels to show.
This gives us more (much?) efficient packing of letters
and we load exact size of the letters.
It looks visually a lot better too!
This wasn't good with normal smapling, but using texelFetch its
great.
