[paragui-users] Const correctness.

[Guillaume Schmid]

Hello,

I am using the PG_Draw functions and noticed that anoyingly, the
`PG_Draw::BlitSurface(SDL_Surface *, const PG_Rect &, SDL_Surface *,
const PG_Rect &)' function does not takes a const as source surface.
Except if the internals of this function does modify the source surface
(Does it ?) I think that it could benefit from being defined as a const.
The same comment goes to all functions of the same kind in the PG_Draw
namespace.

a++
Guillaume.