Implement GLX_SGI_make_current_read

Implement GLX_SGI_make_current_read

Discontinue use of the old GetBuffeSize interface.  Track both the
current read-drawable and the current draw-drawable.  After moving
some context state to via_rednerbuffer, GLX_SGI_make_current_read can
be enabled.

The extension works, but the wincopy test prodcues a black window for
the destination window.  After messing around with the window and
looking at the code, I believe the problem is in the handling of
buffer swap requests on a drawable that isn't the current draw-drawable.