glx: Don't create a shared context if the other context isn't the same kind

glx: Don't create a shared context if the other context isn't the same kind

Each of the DRI, DRI2, and DRISW backends contain code like the
following in their create-context routine:

   if (shareList) {
      pcp_shared = (struct dri2_context *) shareList;
      shared = pcp_shared->driContext;
   }

This assumes that the glx_context *shareList is actually the correct
derived type.  However, if shareList was created as an
indirect-rendering context, it will not be the expected type.  As a
result, shared will contain garbage.  This garbage will be passed to
the driver, and the driver will probably segfault.  This can be
observed with the following GLX code:

    ctx0 = glXCreateContext(dpy, visinfo, NULL, False);
    ctx1 = glXCreateContext(dpy, visinfo, ctx0, True);

Create-context is the only case where this occurs.  All other cases
where a context is passed to the backend, it is the 'this' pointer
(i.e., we got to the backend by call something from ctx->vtable).

To work around this, check that the shareList->vtable->destroy method
is the same as the destroy method of the expected type.  We could also
check that shareList->vtable matches the vtable or by adding a "tag"
to glx_context to identify the derived type.

NOTE: This is a candidate for the 7.11 branch.

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