Mandelbrot: out-of-bounds access to the palette.

Fixes #8812, #10920

At the edge of the fractal, we can get a value between 0 and the number
of iterations. Since the palette is only 256 colors, this was previously
using random colors for any setting with more than 256 iterations, and
ended up crashing the program with very high numbers.

The palette is now looped, which avoids the crash. Looping the palette
allows finer details at the higher zoom levels to be provided using the
full color range, however with some of the palettes this will look quite
noisy.