GETSCREENSIZE change

[Dark Schneider]

IMO it could be a good idea that GETSCREENSIZE returns the size of the current screen, not only the backbuffer.
So if we do an USESCREEN other than -1, then GETSCREENSIZE give us the size of that screen (render target).

This could be a good solution to use render targets more efficiently.

[Kitty Hello]

you can use getspritesize. But yes, I'll think it's a good idea.