[gtk/wip/chergert/glproto] more assertions

[Christian Hergert <chergert src gnome org>]

commit 6c58e8df56ac1b939535500b4f8675b9f782a755
Author: Christian Hergert <chergert redhat com>
Date:   Fri Jan 29 13:07:08 2021 -0800

    more assertions

 gsk/next/gskglrenderjob.c | 4 ++++
 1 file changed, 4 insertions(+)
---
diff --git a/gsk/next/gskglrenderjob.c b/gsk/next/gskglrenderjob.c
index d5e69ec38a..9aa46739c2 100644
--- a/gsk/next/gskglrenderjob.c
+++ b/gsk/next/gskglrenderjob.c
@@ -2891,12 +2891,16 @@ gsk_gl_render_job_visit_blend_node (GskGLRenderJob *job,
       return;
     }
 
+  g_assert (bottom_offscreen.was_offscreen);
+
   if (!gsk_gl_render_job_visit_node_with_offscreen (job, top_child, &top_offscreen))
     {
       gsk_gl_render_job_load_vertices_from_offscreen (job, &node->bounds, &bottom_offscreen);
       return;
     }
 
+  g_assert (top_offscreen.was_offscreen);
+
   gsk_gl_program_begin_draw (job->driver->blend,
                              &job->viewport,
                              &job->projection,