This adds the option to pass a subpixel offset when fetching a glyph
from a font, this offset is currently snapped to thirds of a pixel
(i.e. 0, 0.33, 0.66). This is then used when rasterizing the glyph,
which is then cached like usual.
Note that when using subpixel offsets you're trading a bit of space
for accuracy. With the current third of a pixel offsets you can end
up with up to 9 bitmaps per glyph.
