debugging truetype crash
Fix a few bugs with subpixel shifts, shift_y in particular.

1. In the presence of nonzero shift_x / shift_y,
   stbtt_GetGlyphBitmapBoxSubpixel would return a nonzero-sized bounding
   box for empty glyphs (e.g. spaces). Since such glyphs don't have any
   outlines, the rasterizer wouldn't do anything, resulting in a 1x1-pixel
   image with uninitialized memory.
2. GetGlyphBitmapBoxSubpixel added shift_y then flipped the y axis,
   whereas the rasterizer flipped the y axis then added shift_y.
   Consistently flip-then-add in both places. This also makes the pattern
   of floors/ceils in GetGlyphBitmapBoxSubpixel simpler.
3. The rasterizer added shift_y after multiplying by the vertical
   oversampling factor, instead of before.

Vertical shifts now work much better, in my tests anyway.
Merge branch 'master' into working
Merge branch 'master' of http://github.com/rygorous/stb into working
update version number & contributors
version numbers