apple: Use glapi rather than reinventing the wheel

apple: Use glapi rather than reinventing the wheel

With this change, Apple's libGL is now using glapi rather than implementing
its own dispatch.  In this implementation, two dispatch tables are created:

__ogl_framework_api always points into OpenGL.framework.
__applegl_api is the vtable that is used.  It points into OpenGL.framework
or to local implementations that override / interpose this in OpenGL.framework

The initialization for __ogl_framework_api was copied from XQuartz with some
modifications and probably still needs further edits to better deal with
aliases.

This is a good step towards supporting both indirect and direct rendering
on darwin.

Signed-off-by: Jeremy Huddleston <jeremyhu@apple.com>