1、系统要提供
A、rendering contexts (渲染环境) B、system framebuffers（系统帧缓冲区）
2、iOS does not provide system framebuffers. Instead, iOS extends the framebuffer object provided by OpenGL ES to allow framebuffers that share data with Core Animation.
iOS没有提供system framebuffer，代替的是，iOS扩展了framebuffer，使得framebuffer可以和Core Animation可以共.享数据。
3、Every thread in an iOS application maintains a current context
每一个线程维持一个context，通过[EAGLContext setCurrentContext: myContext];来设置当前线程context
