Add: [OSX] Text layout using the native CoreText API.

By default, the native API will be used instead of ICU, but if ICU is
forced in using configure, it will take precedence.

src/fontcache.h

@@ -64,6 +64,12 @@ public:
 	 */
 	inline int GetUnitsPerEM() const { return this->units_per_em; }
 
+	/**
+	 * Get the nominal font size of the font.
+	 * @return The nominal font size.
+	 */
+	virtual int GetFontSize() const { return this->height; }
+
 	/**
 	 * Get the SpriteID mapped to the given key
 	 * @param key The key to get the sprite for.