Scene panel goes pixelated

[fasteinke]

Strange one ... as an experiment, tried raising --cull-opacity to a very high value, 0.5, and all appeared reasonable until it reached reset alpha time. Immediately the scene window went into "you can't see me!" large pixels, to match the initial checkerboard pattern. You could still move around the model, but with only a monstrous pixel size display of it.

[ArthurBrussee]

I think this might happen if a NaN is hit, which then spreads out in a weird 'pixelated' pattern because of the SSIM convolutions.

The last few commits should fix a NaN when cull-opacity is set outside of the expected range, so might be fixed!