BUG: The ColorMapper's map_screen() method didn't correctly handle the case of a null range when the data array was 2D.  Also, the null range case now sets the value to 0.5 (the middle of the color range) instead of 1.0.
BUG: The ColorMapper's map_screen() method didn't correctly handle the case of a null range when the data array was 2D.  Also, the null range case now sets the value to 0.5 (the middle of the color range) instead of 1.0.

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