Dynamically generated stacked and scaled compound glyphs

[RebeccaRGB]

Allow any stacked or scaled compound of any two (UCSUR) logographs to combine.

Probably don't bother with logographs outside of UCSUR; though if done right in the first place it wouldn't be difficult to add.

Remove existing compound glyphs? Left to judgement call which version looks better (e.g. the existing precomposed kalama-musi is going to look better than the dynamically-generated version).