Add vogl_core_init()
- Explicitly use this instead of relying on global constructors.
  They don't work well with static libraries (can get optimized out).

 http://www.gamedev.net/topic/622861-how-to-force-global-variable-which-define-in-a-static-library-to-initialize/

- Kill vogl_global_initializer.
- Kill global_heap_constructor.
- Add VOGL_CONSTRUCTOR_FUNCTION macro.
- Fix g_number_of_processors check on Linux.