Toggle delta sets on/off

[Lorp]

Offer a toggle on the current glyph to see the graphical effect. It should be particularly useful for corner masters.

It could be useful to offer disabling delta sets on a whole font, via command line or GUI. We can’t use the id of the delta set (the same master is not guaranteed to have the same delta set id in each glyph), so we must use a filter on the tuple. Disabling a delta set by tuple through a whole font on the command line could use an argument such as --disable-delta-set "<tuple-filter>" where tuple-filter specifies the tuple in full, e.g. [1,1] to delete a corner tuple, or [0.755 0.877 0.901] to remove an intermediate tuple.

Care needs to be taken that adding a "disabled" check does not affect export time on the default case, fonts with all delta sets active.

Toggling of delta sets on a single glyph, with immediate visual feedback, was implemented in the 2017 version of Samsa.

[Lorp]

@davelab6