Fix renderer not clearing some GL extension variables at vid_restart

If renderer is compiled into client (USE_RENDERER_DLOPEN=0) and after
start up set r_allowExtension to 0 and run vid_restart, some extension
were still used.

code/sdl/sdl_glimp.c

@@ -308,6 +308,13 @@ static void GLimp_ClearProcAddresses( void ) {
 	QGL_ES_1_1_PROCS;
 	QGL_3_0_PROCS;
 
+	qglActiveTextureARB = NULL;
+	qglClientActiveTextureARB = NULL;
+	qglMultiTexCoord2fARB = NULL;
+
+	qglLockArraysEXT = NULL;
+	qglUnlockArraysEXT = NULL;
+
 #undef GLE
 }