glx: don't destroy context immediately if it's currently bound

According to the GLXDestroyContext() man page, the context should not
immediately be destroyed if it's bound to some thread.  Wait until it's
unbound to really delete it.  The code for doing the later part is
already present in MakeContextCurrent() so no change was needed there.

src/glx/x11/glxcmds.c

@@ -540,6 +540,16 @@ DestroyContext(Display * dpy, GLXContext gc)
    imported = gc->imported;
    gc->xid = None;
 
+   if (gc->currentDpy) {
+      /* This context is bound to some thread.  According to the man page,
+       * we should not actually delete the context until it's unbound.
+       * Note that we set gc->xid = None above.  In MakeContextCurrent()
+       * we check for that and delete the context there.
+       */
+      __glXUnlock();
+      return;
+   }
+
 #ifdef GLX_DIRECT_RENDERING
    /* Destroy the direct rendering context */
    if (gc->driContext) {