Subpixel-positioned text looks blurry, due to bilinear filtering of the
underlying texture. The basic idea here is to stretch the font glyphs
horizontally to increase the sharpness at subpixel positions. The
stretched images need to be smoothed to avoid aliasing artifacts; this
is done in FontFaceFreeType::BoxFilter().
Glyphs are always pixel-aligned vertically, so no vertical oversampling
is needed.
To make this feature comprehensible (I hope!) I've removed the
'subpixelGlyphPositions' flag and replaced it with a couple of values:
'fontSubpixelThreshold' sets the point at which subpixel positioning
kicks on, and 'fontOversampling' controls the amount of stretching.
The default values are 12pt text and 2x oversampling. These are fairly
conservative settings, which should improve small text without wasting
a lot of memory.
Note that when the font hint level is NORMAL (the default), subpixel
positioning and oversampling are both disabled. So, this feature doesn't
change any default behavior, and applies some hopefully sensible values
if the hint level is set to LIGHT or NONE.
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