video_output: fully drain video filters before changing the filter aspect ratio

First we peek the decoded picture from the FIFO. It the aspect ratio has
changed we use the pictures remaining in the filters, as long as they are not
late.
If there is no more picture left in the filters we change the filter and
actually pop the decoded picture from the FIFO.

src/video_output/video_output.c

@@ -1124,7 +1124,7 @@ static picture_t *ThreadGetPrerenderedLocked(vout_thread_sys_t *vout, bool reuse
         if (reuse_decoded && sys->displayed.decoded) {
             decoded = picture_Hold(sys->displayed.decoded);
         } else {
-            decoded = picture_fifo_Pop(sys->decoder_fifo);
+            decoded = picture_fifo_Peek(sys->decoder_fifo);
             if (!decoded)
                 break;
 
@@ -1132,6 +1132,17 @@ static picture_t *ThreadGetPrerenderedLocked(vout_thread_sys_t *vout, bool reuse
             if (!VideoFormatIsCropArEqual(&decoded->format, &sys->filter.src_fmt))
             {
                 // we received an aspect ratio change
+                // drain the last pictures remaining in filters before
+                // using the decoded picture with a new format
+                picture = filter_chain_VideoDrain(sys->filter.chain_static, true);
+                if (picture && DropLate(vout, picture, can_drop_late, paused))
+                {
+                    picture = NULL;
+                    continue; // try again
+                }
+                if (picture)
+                    break;
+
                 // Update the filters with the filter source format with the new aspect ratio
                 video_format_Clean(&sys->filter.src_fmt);
                 video_format_Copy(&sys->filter.src_fmt, &decoded->format);
@@ -1141,6 +1152,9 @@ static picture_t *ThreadGetPrerenderedLocked(vout_thread_sys_t *vout, bool reuse
 
                 ThreadChangeFilters(vout);
             }
+
+            // we are going to use this decoded picture, pop it for good
+            decoded = picture_fifo_Pop(sys->decoder_fifo);
         }
         reuse_decoded = false;