Clear glyph pixels when all channels differ below a threshold

[Tyriar]

Fixes #3892
Also helps with #3877

Before:

After:

Main differences:

• A pixel in the top left of W in "WARN" is gone
• A pixel in the bottom right of k in "Work"
• Some of the AA around the green text on yellow text disappears, zoomed out I can't see a difference on 1080p 24" monitor

I tried initially by checking the contrast ratio which is relatively expensive to calculate and we would also need to compare the hue in that case.