register slope contrast cvar so it actually works

src/hardware/hw_main.c

@@ -5794,6 +5794,7 @@ void HWR_AddCommands(void)
 	CV_RegisterVar(&cv_glskydome);
 	CV_RegisterVar(&cv_glspritebillboarding);
 	CV_RegisterVar(&cv_glfakecontrast);
+	CV_RegisterVar(&cv_glslopecontrast);
 	CV_RegisterVar(&cv_glshearing);
 	CV_RegisterVar(&cv_glshaders);