Add: [Win32] Text layout using the native Windows Uniscribe library.

Uniscribe is sometimes producing different results compared to ICU, especially
when RTL and LTR content is mixed. Comparing the results to other programs
(like editors or web browsers) leads me to believe that the result are at least
not worse than ICU and possibly better.

src/fontcache.h

@@ -143,6 +143,11 @@ public:
 	{
 		return this->parent != NULL;
 	}
+
+	/**
+	 * Is this a built-in sprite font?
+	 */
+	virtual bool IsBuiltInFont() = 0;
 };
 
 /** Get the SpriteID mapped to the given font size and key */