remove sub_position as it makes everything more complex. Instead draw on
position.
add development flag which enables grid to be drawn so I can see how
units are drawn in respect to their cells which I need to know if I want
to get foot based unit positioned correctly on a single cell
- this breaks moving behavior for now
- there is no concept anymore of a sub-cell
- all units are positioned at the 'position' point which is from now on
  regarded as the center of a unit
- drawing has been fixed to respect 'position' to be center of unit
- units can have a different size opposed to the tile size, although it
  is not really used yet. This is needed for infantry which has tiles of
  32x32px but the units themselves are ~ 8x8px and they all should be
  able to be placed on the same 32x32px cell.
- add DEV compiler flags to show target of unit, next move position and
  the current position. Later one is needed to know how the unit was
  being drawn in respect to the actual position