pan_state: don't modify mapper range if no change
pan_state: don't modify mapper range if no change

The way panstate is implemented may accumulate rounding error
This is particularly important for zoom_to_mouse feature,
on only one axis, which is implemented with a 1 axis only panstate
even if prev[1] == next[1], the value axis will increase slightly
at each mouse wheel scroll.

Signed-off-by: Pierre Tardy <bb5a2a16eebb4a48138fca46fede24b3c8d635fe@intel.com>

burnpanck/chaco,burnpanck/chaco,tommy-u/chaco,ContinuumIO/chaco,ContinuumIO/chaco,tommy-u/chaco,burnpanck/chaco,tommy-u/chaco,ContinuumIO/chaco,ContinuumIO/chaco