We only want to move to float for the internal 0..1 positions which are
used to draw to the screen (to reduce how much data we hold on to). But for
the data we need to calculate those 0..1 positions, we want better
precision for large numbers. Use double for all of those.
This is meant to contain information about XY coordinates that we can
store in a normalized form. That allows them to be reused across different
scales without regenerating them.
It also includes a back-pointer to the model index so that we can pull out
the real object when necessary for drawing.
