Add ability to drop invisible frames from a test

This allows us to drop invisible frames before an S or E frame.
As of this commit, the SFrameTest passes regardless of the placement
of an invisible frame. This commit does not seem to fix all cases for
the E-frame, so this will still need to be investigated in a followup.

Change-Id: I631e29031afa427bfd54ff0e4793665f85c1bd1c
diff --git a/aom/aom_encoder.h b/aom/aom_encoder.h
index 94849bd..875e15c 100644
--- a/aom/aom_encoder.h
+++ b/aom/aom_encoder.h
@@ -159,6 +159,8 @@
        * Only applicable when "output partition" mode is enabled. First
        * partition has id 0.*/
       int partition_id;
+      /*!\brief size of the visible frame in this packet */
+      size_t vis_frame_size;
     } frame;                            /**< data for compressed frame packet */
     aom_fixed_buf_t twopass_stats;      /**< data for two-pass packet */
     aom_fixed_buf_t firstpass_mb_stats; /**< first pass mb packet */