Default number of colourmap levels

[jdtournier]

Discussion following on from #32: how many colourmap levels should be default to? I've set the default to 32 on the grounds that it give adequate image quality while keeping the frame size small, which helps with responsiveness. But it is clearly not as good as a higher value. My personal opinion is that mrpeek is not geared to high-quality graphics, and we should prioritise responsiveness if possible. Others will no doubt have other opinions - happy to go with the consensus on that one.