2 masters on COLRv1 axes does not allow default in the middle.

[petrvanblokland]

Currently each COLRv1 axes has 2 masters. That causes limitations when selecting a default in the middle (e.g. with horizontal and vertical positioning). As mentioned before, a work around is to place multiple shapes on the same layer, so it is less of a problem that the half of the circle on the corner cannot be reached. I tried adding default masters to the X and Y axes, but that increases the number of color masters from 64 (2^6) to 729 (3^6), just for the color masters. Note that Bitcount has a large glyphset, so this adds a lot to the volume (and compile time)