Add support for X11+EGL.

Add support for X11+EGL.
The changes are made with minimalism and clarity in mind:
- Either the functions are common and are kept in _x11.c files
- Either the functions have small differences and dealt with #ifdef
- Either the functions are largely different, and split in:
  - Specialized portable code in egl/
  - Specialized GLX code in _x11_glx.c files

Using EGL or GLX is decided at compile time
(CMake FREEGLUT_GLES1 or FREEGLUT_GLES2 option enabled)..

git-svn-id: svn+ssh://svn.code.sf.net/p/freeglut/code/trunk/freeglut/freeglut@1196 7f0cb862-5218-0410-a997-914c9d46530a