It's amazing how long it can take to notice terrible design.

fromScreen() is supposed to convert a position given in pixel coordinates to sky coordinates ( (RA,Dec) or (Az,Alt) ). Somehow, all this time, it's been taking dX, dY arguments, which are the *angular* offset from the focus position, in radians. In other words, before calling fromScreen(), you first had to do half the work of conversion!
Wow.

Anyway, it's fixed now. fromScreen() now takes a QPointF: the screen position to be converted.

Also fixed bizarre behavior of Equirectangular projection.

Still to do:
get all parts of the sky to be drawn in that projection system