CONFIG_INSPECTION: fix showable and overlay frame behavior

Remove show existing frame from decode order, and add showing of
an overlay frame that doesn't routinely update any buffer.

Change-Id: I9176bfa53a0542b51891fa510f5d62f102a4e8c5
diff --git a/av1/decoder/inspection.h b/av1/decoder/inspection.h
index 51e6d19..0c6f3ad 100644
--- a/av1/decoder/inspection.h
+++ b/av1/decoder/inspection.h
@@ -75,6 +75,7 @@
   // TODO(negge): add per frame CDEF data
   int delta_q_present_flag;
   int delta_q_res;
+  int show_existing_frame;
 };
 
 void ifd_init(insp_frame_data *fd, int frame_width, int frame_height);