The T2K rasteriser has a flag that controls the quality of greyscale glyphs.
The actual difference is not that great, but it also makes a difference to
peformance. The values are LOW, MEDIUM, HIGH, EXTREME.
We've always used HIGH. Under some transforms, that are typical of animations,
performance is as important as quality, so we should use LOW in those cases.
Could also consider EXTREME for the most typical desktop UI cases.