[SDL] new alpha blit semantics

[Michael Chen]

What do you folks think of allowing 8-bit palettized surfaces to have 
alpha in the palette colors?
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.
So, maybe if an 8-bit surface has SRCALPHA set to 0 (per-pixel), the 
alpha values in the palette colors would be used; if SRCALPHA is set to 
a constant (per surface), they would not be used; if SRCALPHA not set, 
treat as RGB.