gobj: fix tests crash due to ShaderBuffer context not cleared at GSG death

panda/src/gobj/preparedGraphicsObjects.cxx

@@ -1371,6 +1371,8 @@ release_all_shader_buffers() {
        ++bci) {
 
     BufferContext *bc = (BufferContext *)(*bci);
+    ((ShaderBuffer *)bc->_object)->clear_prepared(this);
+    bc->_object = nullptr;
     _released_shader_buffers.insert(bc);
   }