Time for action – initializing OpenGL ES

Let's rewrite our GraphicsManager to initialize an OpenGL ES context:

1. Modify jni/GraphicsManager.hpp by performing the following:
   • Include EGL/egl.h to bind OpenGL ES to the Android platform and GLES2/gl2.h to render graphics
   • Add a method stop() to unbind the OpenGL rendering context and free graphics resources when you're leaving the activity
   • Define EGLDisplay, EGLSurface, and EGLContext member variables, which represent handles to system resources, as shown here:

     ...
     #include "Types.hpp"
     
     #include <android_native_app_glue.h>
     #include <GLES2/gl2.h>
     #include <EGL/egl.h>
     ...
     
     class GraphicsManager {
     public:
         ...
         status start();
         void stop();
         status update();
     
     private:
         ...
         int32_t mRenderWidth; int32_t mRenderHeight;
         EGLDisplay mDisplay; EGLSurface mSurface; EGLContext mContext;
     
         GraphicsElement* mElements[1024]; int32_t mElementCount;
     };
     #endif

2. Reimplement jni/GraphicsManager.cpp...