xlib: fix glXGetCurrentDisplay() failure

xlib: fix glXGetCurrentDisplay() failure

glXGetCurrentDisplay() has been broken for years and nobody noticed until
recently.  This change adds a new XMesaGetCurrentDisplay() that the GLX
emulation API can call, just as we did for glXGetCurrentContext().

Tested by hacking glxgears to call glXGetCurrentContext() before and
after glXMakeCurrent() to verify the return value is NULL beforehand and
the same as the opened display afterward.

Also tested by Tom Hudson with his tests programs.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=100988
Cc: mesa-stable@lists.freedesktop.org
Tested-by: Tom Hudson <tom.hudson.phd@gmail.com>
Signed-off-by: Brian Paul <brianp@vmware.com>