drisw: Fix shared memory leak on drawable resize

XDestroyImage will mark the segment as to-be-destroyed, but it will
persist until we detach it, and we weren't doing so.

Cc: mesa-stable@lists.freedesktop.org
Gitlab: https://gitlab.freedesktop.org/mesa/mesa/issues/121
Reviewed-by: Adam Jackson <ajax@redhat.com>

src/glx/drisw_glx.c

@@ -56,6 +56,8 @@ XCreateDrawable(struct drisw_drawable * pdp, int shmid, Display * dpy)
    if (pdp->ximage) {
       XDestroyImage(pdp->ximage);
       pdp->ximage = NULL;
+      if ((pdp->shminfo.shmid > 0) && (shmid != pdp->shminfo.shmid))
+         XShmDetach(dpy, &pdp->shminfo);
    }
 
    if (!xshm_error && shmid >= 0) {