I broke this, back in:

    commit a48dadc9
    Author: Adam Jackson <ajax@redhat.com>
    Date:   Mon Mar 21 11:59:29 2011 -0400

	glx: Reimplement context tags

In that, I changed the glx client state to not explicitly track the list
of current contexts for the client (since that was what we were deriving
tags from).  The bug was that I removed the code for same from
glxClientCallback without noticing that it had the side effect of
effectively de-currenting those contexts, so that ContextGone could free
them.  So, if you had a client exit with a context still current, the
context's memory would leak.  Not a huge deal for direct clients, but
viciously bad for indirect, since the swrast context state at the bottom
of Mesa is like 15M.

Fix this by promoting Bool isCurrent to ClientPtr currentClient, so that
we have a back-pointer to chase when walking the list of contexts when
ClientStateGone happens.

v2: Explicitly call __glXFreeContext on the ClientStateGone path.  Our
current context might be one we got from EXT_import_context and whose
creating client has since died.  Without the explicit call, the creating
client's FreeClientResources would not free the context because it's
still current, and the using client's FreeClientResources would not free
the context because it's not an XID it created.  This matches the logic
from a48dadc9

.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Signed-off-by: Adam Jackson <ajax@redhat.com>