Extend OpenGl_Context to provide GL2.0 core functionality
Added 'glext.h' header provided by Khronos group with definitions
and GL functions' types.
Added OpenGl_GlCoreXX structures with function list
to appropriate GL core functionality.
Fixed memory leak in OpenGl_Context destructor.
Eliminate inclusions of gl.h header
Use OpenGl_GlCore11.hxx instead.
Removed obsolote M_PI redefinitions.
Slightly cleaned up included headers.
Reuse definitions from glext.h
OpenGl_ArbVBO and OpenGl_ExtFBO originally provide own definitions
for OpenGL extensions.
