[SDL] Const correctness for SDL video

[Christophe Cavalaria]

Martin Storsjö wrote:

> Of course, I won't try to say anything about whether adding casts to the
> code interfacing to e.g. SDL or actually cleaning up the const-ness of SDL
> is easier...

Cleaning SDL is easier because once someone does that, everybody who uses it
and had that const problem before will have that problem mostly fixed for
them :) It's a case where doing it isn't probably worth it for one person
but is worth it for all users.