gbm: Add gbm_surface interface

gbm: Add gbm_surface interface

The idea here is to be able to create an egl window surface from a
gbm_surface.  This avoids the need for the surfaceless extension and
lets the EGL platform handle buffer allocation, while keeping the user
in charge of somehow presenting the buffers (using kms page flipping,
for example).

gbm_surface_lock_front_buffer() locks a surface's front buffer and
returns a gbm bo representing it.  This bo should later be returned
to the gbm surface using gbm_surface_release_buffer().