[ab] a rectangle generic

[Luke J Crook]

Samium Gromoff wrote:
 > The rectangle generic could produce the rectangle of the surface.
> 
> The example above is really dubious, it should have been something like
> 
> 	(sdl:draw-box (sdl:rectangle surface) :surface surface)
> 


That sounds reasonable. Trouble is that sdl:rectangle creates a new
rectangle.

How about:

GET-CLIP-RECT - We already have this one defined. It gets the clip
rectangle for the surface.
GET-SURF-RECT - Get the rectangle that describes the surface
GET-CELL-RECT - Get the rectangle used for drawing the current cell in a
sprite sheet.

- Luke