Actually make the Contrast-O-Vision filter WCAG AA-compliant

[tigt]

The dirty secret: right now, it has a lot of false positives/negatives. I've been trying to to fix it for months, but even my reduced test cases baffle me.

I irresponsibly put this online in the hope that somebody calls me out on it and inadvertently points me towards fixing it.

[tigt]

Possible leads

• The color-interpolation-filters conversion is broken in at least Blink, which means the native<feColorMatrix type="luminanceToAlpha"> may not be converting luminance correctly.

• The luminance is clamped badly (i.e. it overflows past the possible alpha values 0 and 1). This could require anything from decreasing the gamma to rigging up a homebrew version with <feComponentTransfer>.

• Maybe both, sadly.