Replace BlitMult with overlay blending

[cxong]

As the game was converted from palette-swapping to RGB blitting #129, the multiply blend mode was used. It's become apparent that this was the wrong choice of blend mode. Multiply can only darken the input image, producing subdued results.

Instead the blend mode should be overlay. For example, overlay against pure red will leave blacks black, whites white, but will colourise all the greys into shades of red.

Perhaps overlay can also replace BlitTint, which is attempting to do the same thing.

This will be necessary as we add more character faces; the subdued effect is especially noticeable on the characters.

One thing to note is that BlitMult may have been chosen because it can be efficiently implemented, but its replacement shouldn't be used to blit at all, only to pre-process images, ala #440

[cxong]

Unfortunately the old palette swapping isn't quite overlay; multiply is still the closest effect.