prime the fontstash glyph cache before the game starts

Not even remotely necessary, but I wanted to do it anyway.
update to latest version of fontstash.h

Corresponds with fontstash version
@da9cf9249156840b3485096832f8234118d1272f
(2014-03-22)

from https://github.com/memononen/fontstash
fix bug in fontstash atlas-full error handling
set a font-stash error handling callback
add a fonsDrawDebug call at the end of render() (currently disabled)
fontstash: compute string end earlier so it can be reused by fonsTextBounds
fontstash: fix use of 'end' parameters of fonsDrawText and fonsTextBounds
fontstash: add enum values to mask alignment flag groups
implement the display for a help screen