st/xlib: Support unlimited number of display connections

st/xlib: Support unlimited number of display connections

There is a limit of 10 display connections, which was a
problem for apps/tests that were continuously opening/closing display
connections.

This fix uses XAddExtension() and XESetCloseDisplay() to keep track
of the status of the display connections from the X server, freeing
mesa-related data as X displays get destroyed by the X server.

Poster child is the VTK "TimingTests"

Tested-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Brian Paul <brianp@vmware.com>