[SDL] 2D API evolution (SDL 1.3/2.0)

[Sam Lantinga]

> For each blend mode you have one case statement.

>         case PYGAME_BLEND_ADD: {
>             BLEND_TOP_4;
>             BLEND_ADD4(*src,*dst);
>             BLEND_START_GENERIC;
>             BLEND_ADD(sR, sG, sB, sA, dR, dG, dB, dA);
>             BLEND_END_GENERIC;
>             break;
>         }

SDL's blitters are mostly incredibly tuned C.  Do you get really good
performance from this method?

	-Sam Lantinga, Senior Software Engineer, Blizzard Entertainment