src/xvba_video_glx.c: properly cleanup on shutdown of render_thread()

The render thread allocates a new render_context, but never destroys it
when shutting down. It possibly leaks on closing.

src/xvba_video_glx.c

@@ -187,6 +187,8 @@ static void *render_thread(void *arg)
     Display * const x11_dpy = driver_data->x11_dpy_local;
 #endif
 
+    ASSERT(obj_output->render_context);
+
     GLContextState old_cs;
     obj_output->render_context = gl_create_context(
         x11_dpy,
@@ -247,6 +249,8 @@ static void *render_thread(void *arg)
             break;
         }
     }
+
+    gl_destroy_context(obj_output->render_context);
     gl_set_current_context(&old_cs, NULL);
     return NULL;
 }