GetSprite() is used to gain exclusive access to one of the eight hardware
sprites.  Once you have gained control of a hardware sprite, it can no
longer be allocated by the GELs system for use as a VSprite.  The call is
made like this:

    struct SimpleSprite *sprite;
    SHORT               number;

    if (-1 == (sprite_num = GetSprite(sprite, number)))
        return_code = RETURN_WARN;  /* did not get the sprite */

The inputs to the GetSprite() function are a pointer to a SimpleSprite
structure and the number (0-7) of the hardware sprite to be accessed, or
-1 to get the first available sprite.

A value of 0-7 is returned if the request was granted, specifying which
sprite was allocated.  A returned value of -1 means the requested sprite
was not available.  If the call succeeds, the SimpleSprite data structure
will have its sprite number field filled in with the appropriate number.