Misc code cleanups, little SkeletalSprite loading improvement.

- Removed some unused member variables.

- The BBGE_BUILD_WIDESCREEN define is now gone.

- Added an TiXMLDocument cache to prevent parsing the same file
once for each entity on the map with the same skeletal.

- Removed Lua func entity_warpToPathStart, which was essentially a no-op
because Entity::followingPath was always NULL. Removed related code.

- Set texture wrap only when required.

(Some changes taken from https://bitbucket.org/mattbierner/ios-aquaria,
special thanks for changeset 72d6460d9e60)

BBGE/SkeletalSprite.h

@@ -259,6 +259,7 @@ public:
 	std::string filenameLoaded;
 
 	static std::string animationPath, skinPath, secondaryAnimationPath;
+	static void clearCache();
 	
 protected:
 	bool frozen;