mapi: Add support for bridge mode.

mapi: Add support for bridge mode.

In bridge mode, mapi no longer implements glapi.h.  It becomes a user of
glapi.h.  Imagine an app that uses both libGL.so and libGLESv2.so.
There will be two copies of glapi in the app's memory.  It is possible
that _glapi_get_dispatch does not return what _glapi_set_dispatch set,
if they access different copies of the global variables.  The solution
to this situation to build either one of the libraries as a bridge to
the other.  Or build both libraries as bridges to another shared
glapi library.