Move GLES20 source into standard locations
Move Khronos headers into their respective components, to be exported by each.
Remove hostthreadadapter as nothing outside of the vghwapiwrapper, which now contains the code, needs it
#ifndef _CPLEXT_H
#define _CPLEXT_H
#if __GNUC__ >= 3
#pragma GCC system_header
#endif
#define CPLPAGE_MOUSE_BUTTONS 1
#define CPLPAGE_MOUSE_PTRMOTION 2
#define CPLPAGE_MOUSE_WHEEL 3
#define CPLPAGE_KEYBOARD_SPEED 1
#define CPLPAGE_DISPLAY_BACKGROUND 1
#endif