split gl_sky.c into gl_sky.c and gl_sky_clip.c (should have done this in the

first place, but I didn't realise how much code would go into sky clipping).

include/glquake.h

@@ -127,6 +127,7 @@ extern	PROC glVertexPointerEXT;
 #define SKYSHIFT		7
 #define	SKYSIZE			(1 << SKYSHIFT)
 #define SKYMASK			(SKYSIZE - 1)
+#define SKY_TEX			2000	// Quake 2 environment sky
 
 #define BACKFACE_EPSILON	0.01