fix performance problems due to SCREEN_WIDTH being evaluated many times per frame, leading to many Lua compilations per frame

stepmania/src/ScreenDimensions.cpp

@@ -1,6 +1,8 @@
 #include "global.h"
 #include "ScreenDimensions.h"
 
+ThemeMetric<float> SCREEN_WIDTH("Common","ScreenWidth");
+ThemeMetric<float> SCREEN_HEIGHT("Common","ScreenHeight");
 
 /*
  * (c) 2001-2002 Chris Danford

stepmania/src/ScreenDimensions.h

@@ -4,9 +4,10 @@
 #define SCREEN_DIMENSIONS_H
 
 #include "ThemeManager.h"
+#include "ThemeMetric.h"
 
-#define	SCREEN_WIDTH	THEME->GetMetricF("Common","ScreenWidth")
-#define	SCREEN_HEIGHT	THEME->GetMetricF("Common","ScreenHeight")
+extern ThemeMetric<float> SCREEN_WIDTH;
+extern ThemeMetric<float> SCREEN_HEIGHT;
 
 #define		SCREEN_LEFT		(0)
 #define		SCREEN_RIGHT	(SCREEN_WIDTH)