ref_soft: de-duplicated symbols

- Compiles correctly with GCC 10 now

ref_soft/r_bsp.c

@@ -24,8 +24,6 @@ Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA  02111-1307, USA.
 //
 // current entity info
 //
-qboolean		insubmodel;
-								// the currently rendering entity
 vec3_t			r_entorigin;	// the currently rendering entity in world
 								// coordinates
 

ref_soft/r_context.c

@@ -17,14 +17,11 @@ GNU General Public License for more details.
 
 ref_api_t      gEngfuncs;
 ref_globals_t *gpGlobals;
-ref_instance_t RI;
 gl_globals_t tr;
 ref_speeds_t r_stats;
 byte *r_temppool;
 cvar_t *gl_emboss_scale;
-cvar_t *r_drawentities;
 cvar_t *r_norefresh;
-cvar_t *vid_gamma;
 cvar_t	*vid_brightness;
 viddef_t vid;
 static void GAME_EXPORT R_ClearScreen( void )

ref_soft/r_glblit.c

@@ -1,4 +1,5 @@
 #include "r_local.h"
+#define APIENTRY_LINKAGE static
 #include "../ref_gl/gl_export.h"
 
 struct swblit_s