Add a new interface between softpipe and the window system / buffer manager.

Add a new interface between softpipe and the window system / buffer manager.

This interface is defined by softpipe and any window system (eg i915pipe)
wishing to use softpipe is required to implement the interface.
Currently the interface is all about buffer management.

Generalizing, each pipe driver will advertise an interface in a similar
spirit to this, and again any window system driver wishing to use that
rendering pipeline will have to implement the interface it defines.  It
clearly isn't a one-way street however, as softpipe could just do its own
buffer management with malloc.  The interaction with a buffer manager is
desired to allow us to exercise the hardware swapbuffers functionality of
the i915pipe driver, and also to get a feel for the way hardware drivers
which really need a buffer manager will work.