Interleave error from forcing <method> storage

[solidpixel]

Analysis shows that we get increased cache pressure in compute_lowest_and_highest_weight because these two:

float error_from_forcing_weight_down[60];
float error_from_forcing_weight_either_way[60];

... are stored as separate arrays and accessed inside a hot loop. We always access the same index in each array at the same time, so we should store this as a single array of structs to reduce cache pressure.

[solidpixel]

Rejecting. This helps on older CPUs, but shows no advantage on current generation Core processors (tested on a Core-i5).