Fix sincos table under/overflow

[solidpixel]

The current code computes a sincos table index based on the ideal weight for each texel, which is computed based on the current ideal endpoints. Ideal endpoints can be closer to the mean value than the principal axis projection for some texels and, for these texels, the ideal weights will be outside of the [0,1] range. The current code doesn't allow for this, which results in computed sincos table indices that end up wrapping inside the table bounds.

As part of fixing this I've replaced the float-to-int conversion that was implemented via float bit-twiddling with a straight round-to-nearest, which is faster on modern hardware.

Fixing this does change weight selection, and image PSNR changes are noisy in both directions, so regenerated test reference scores are required.