CrossApp source structure

This directory is the source directory of the engine and its two main directories

CrossApp.h:

All engine features require a header file to be included, and for developers who develop apps using CrossApp, the underlying system differences are blocked, and if cross-platform is required, the header file under this directory will contain the corresponding header file in the CrossApp directory.

CrossApp\platform:

Judge the current operating system with a bunch of s if macros, and the header files under the corresponding system for different systems. CrossApp is based on OPENGL ES (a subset of OPENGL, dedicated to mobile devices), so the underlying image and animation is already cross-platform, so there are not many real cross-platforms in cocos2d (not many head files under platform), and some different platforms, such as the message loop response mechanism, need to cross-platform in different ways