Fixed problem with inverted image rendering

Hi,

Started trying to do some work and realised that the new colourMap() function and associated code in reDraw() caused another bug. If you invert an image, the inverted image does not display correctly.

Problem was when image is inverted it produces floats not ints. I've changed the reDraw() function to cast to floats. Means ints get a decimal point and zero, but floats are unchanged.