glgsg: fix wrong shadow buffer host when rendering scene to buffer

Fixes #890

panda/src/glstuff/glGraphicsStateGuardian_src.cxx

@@ -8303,6 +8303,10 @@ make_shadow_buffer(LightLensNode *light, Texture *tex, GraphicsOutput *host) {
     flags |= GraphicsPipe::BF_size_square;
   }
 
+  if (host != nullptr) {
+    host = host->get_host();
+  }
+
   CLP(GraphicsBuffer) *sbuffer = new CLP(GraphicsBuffer)(get_engine(), get_pipe(), light->get_name(), fbp, props, flags, this, host);
   sbuffer->add_render_texture(tex, GraphicsOutput::RTM_bind_or_copy, GraphicsOutput::RTP_depth);
   get_engine()->add_window(sbuffer, light->get_shadow_buffer_sort());