refactor: #define DEAD_HEIGHT -8

From the research conducted in
https://github.com/ioquake/ioq3/issues/794
we can probably conclude that the `self->r.maxs[2] = -8;`
in `player_die()` is related to `pm->maxs[2] = -8;`
in `PM_CheckDuck()`. Let's make this clear with a named constant.

code/game/bg_public.h

@@ -52,6 +52,7 @@ Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
 #define	DEFAULT_VIEWHEIGHT	26
 #define CROUCH_HEIGHT		16
 #define CROUCH_VIEWHEIGHT	12
+#define DEAD_HEIGHT			-8
 #define	DEAD_VIEWHEIGHT		-16
 
 //