[SDL] Const correctness for SDL video

[Ryan C. Gordon]

> Won't that break binary compatibility in SDL 1.2?

Not in these cases. You can flag data passed to functions as const and 
not break binary compatibility...C function signatures don't change, and 
all you've done is guarantee that you won't change the data passed in to 
the function, and previously-compiled code would have assumed you might 
do so and have coded appropriately.

C++ is a different story: adding a "const" changes the function 
signature and then your program won't be able to find the function 
without a recompile. SDL's public API is all C, though...there's a 
little C++ in the BeOS port, but none of that is publicly exposed.

--ryan.