The resizing and movement mechanics for the avatar crop can be finicky,
because:
1. The crop resets to the last valid crop when you drag out of range, so
that a somewhat quick drag does not go to the edge.
2. A corner drag only works in one direction, as the use of
eval_radial_line is not correct.
3. A side drag acts the same as a corner drag.
4. A slow drag won't move the crop, especially if the widget is larger.
All these issues are fixed with this partial rewrite of the resize and
movement logic:
1. A drag out of range will result in a size that is guaranteed to touch
the edge, making a crop to the edge much easier.
2. Instead of fixing eval_radial_line, we just "bind" one side to the
pointer position, making both directions act the same for a corner
drag.
3. A side drag now scales around the center for the orthogonal side.
4. Rounding errors in calculating the previous delta are prevented.
44e1a1228f