GRAPHICS: Improve TTF rendering on transparent surfaces.

[OMGPizzaGuy]

Update color blending to match blending found in ManagedSurface. TTF rendering assumed the alpha channel of the surface was completely opaque. ULTIMA8 TTF rendering attempts to use a transparent surface and became near-illegible for text on scrolls. This change will affect TTF rendering on surfaces that are not already painted.

[mduggan]

This looks good to me. I tried to find other uses of this code to see if there's any potential side-effects but I didn't find any place I thought it could cause any change other than U8. I may have missed something though so if anyone else wants to comment please feel free.

[criezy]

Where do those formula come from?
My understanding of color blending is that it should be:

double sAn = (double)sA / 255.0;
double dAn = (double)dA / 255.0;
dR = static_cast<uint8>(sR * sAn + dR * dAn * (1.0 - sAn) / (sAn + dAn * (1.0 - sAn)));
dG = static_cast<uint8>(sG * sAn + dG * dAn * (1.0 - sAn) / (sAn + dAn * (1.0 - sAn)));
dB = static_cast<uint8>(sB * sAn + dB * dAn * (1.0 - sAn) / (sAn + dAn * (1.0 - sAn)));
dA = static_cast<uint8>(sA + dA * (1.0 - sAn));

This also means there is no need for a special case if dA is 0.
And you can of course optimise that code a bit by computing dAn * (1.0 - sAn) only once instead of 6 times.

[OMGPizzaGuy]

Mostly followed suit with managed_surface.cpp:487-490
Figured if case for dA is 0 might be better to avoid calculations, but perhaps not.
Destination alpha... I just wasn't sure the best call.

[OMGPizzaGuy]

OK, looking at https://en.wikipedia.org/wiki/Alpha_compositing#Alpha_blending
If I'm understanding it correctly, what I used would be correct for dA being 255, but not complete enough for any other value.
I'll take another swing at this.

[OMGPizzaGuy]

Yep, the suggested code was great.
I went with calculating a normalized output alpha and converting that to a byte at the end.
I thought it would be a little more clear to anyone reading the code.

[criezy]

Thank you. It looks good to me now.