Provide some form of cacheing support for font generation

[robmeek]

e.g. for accelerating real-time/in-app font (re)generation.
– Don’t unnecessarily regenerate entire font files, when parts have not changed – esp. SimpleGlyphDescription objects. Cache such objects (in memory) and only regenerate them if changed.

[robmeek]

Maybe add a “dirty” flag to IContourGlyph