video: take a deferred view count shrink before releasing its frame

The threaded wrapper applied a deferred shrink after
video_thread_ring_complete(), so a main thread leaving
video_thread_wait_idle() could use the driver while the video thread
was still freeing its view chains. Apply it while the frame's slot is
still held; the harness now checks the slot was held when the shrink
arrived.

gfx/video_thread_wrapper.c

@@ -3289,9 +3289,11 @@ static void video_thread_loop(void *data)
             thr->frame.mem_busy |= 1u << frame_mem;
             retro_atomic_store_release_int(&thr->frame.mem[frame_mem].busy, 1);
          }
-         video_thread_ring_complete(thr);
+         /* Before the slot goes: whoever waited for an idle ring may
+          * use the driver the moment it sees one */
          if (thr->views_shrink_left && !--thr->views_shrink_left)
             video_thread_views_apply(thr, thr->views_shrink_to);
+         video_thread_ring_complete(thr);
 
          if (tex_retire)
          {

samples/gfx/threaded_video/threaded_video_test.c

@@ -5894,7 +5894,9 @@ static void vshrink_set_view_count(void *data, unsigned count)
       vshrink_wait_cmd();
    }
    vshrink_count = count;
-   vshrink_note('S', count, 0);
+   /* Whether a frame's slot was still held when it reached the driver */
+   vshrink_note('S', count, VIDEO_THREAD_RING_BUSY_OF(
+            retro_atomic_load_acquire_int(&vshrink_thr->frame.state)));
 }
 
 static void vshrink_get_poke(void *data, const video_poke_interface_t **iface)
@@ -6008,8 +6010,14 @@ static void vshrink_case(const char *name, video_frame_info_t *info,
       CHECK(s >= 0 && s < f, "%s: the grow to %u reached the driver at "
             "%d, after the frame queued ahead of it (%d)", name, to, s, f);
    else
+   {
       CHECK(s > f, "%s: the shrink to %u reached the driver at %d, "
             "before the frame queued ahead of it (%d)", name, to, s, f);
+      CHECK(s >= 0 && vshrink_log[s].views,
+            "%s: the shrink to %u reached the driver after its frame's "
+            "slot was released, with the ring already reading idle",
+            name, to);
+   }
    f = vshrink_find('F', last + 1);
    CHECK(f >= 0 && vshrink_log[f].count == to,
          "%s: the frame after the count change drew at count %u, wanted %u",