[SDL] new alpha blit semantics

[Mattias Engdegård]

>What do you folks think of allowing 8-bit palettized surfaces to have 
>alpha in the palette colors?

There are lots of tricks you can do with 8-bit surfaces and lookup tables;
SDL can only cover the very basics. The one you describe is interesting
and occasionally useful (and both TGA and PNG support per-index alpha values
for colormapped images).

>One example of this feature being useful would be so that nice font 
>libraries like FreeType 2 modified SDL_ttf could store 8-bit surfaces 
>for each character, and change the palette to change colors, instead of 
>allocating a 32-bit RGBA surface and having to render from 8-bit to RGBA 
>manually every time.

Last time I had to do this I rendered the font in 8-bit greyscale (where
0 = transparent, 255 = opaque, essentially just rendering the alpha
channel of the glyphs) and then used a look-up table for each colour.
No doubt SDL_ttf can be persuaded to do this.

Anyway, since it isn't hard to write platform-independent code that does
this in the application that needs it (or maybe in a library like SDL_ttf),
I don't think it's necessary to put it into SDL, unless someone
bribes^Wpersuades me otherwise :-)