Negative accumulation on the right-hand side of the accumulation bitmap
would wrap around to the left, causing a negative start for certain
lines which resulted in horizontal streaks of lower alpha values.
Prevent this by not writing out of bounds :^)
Fixes#18394
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.
The custom TTF path rasterizer is actually generic enough for it to be
used for other fonts. To make this more clear, it now lives on its own
in the "Font" directory.
