matroskamux: Do caps renegotiation when it only adds fields

Matroskamux can accept caps renegotiation if the new caps is a
superset of the old one, meaning upstream added new info to
the caps.

Same logic as a5f22f03aa25b04726f78ae619f40b3b648f7d48 in qtmux.

gst/matroska/matroska-mux.c

@@ -981,9 +981,10 @@ gst_matroska_mux_video_pad_setcaps (GstPad * pad, GstCaps * caps)
 
   if ((old_caps = gst_pad_get_current_caps (pad))) {
     if (mux->state >= GST_MATROSKA_MUX_STATE_HEADER
-        && !gst_caps_is_equal (caps, old_caps)) {
+        && !gst_caps_is_subset (caps, old_caps)) {
       GST_ELEMENT_ERROR (mux, STREAM, MUX, (NULL),
-          ("Caps changed are not supported by Matroska"));
+          ("Caps changes are not supported by Matroska\nCurrent: `%"
+              GST_PTR_FORMAT "`\nNew: `%" GST_PTR_FORMAT "`", old_caps, caps));
       gst_caps_unref (old_caps);
       goto refuse_caps;
     }
@@ -2479,7 +2480,7 @@ gst_matroska_mux_release_pad (GstElement * element, GstPad * pad)
     if (cdata->pad == pad) {
       /*
        * observed duration, this will remain GST_CLOCK_TIME_NONE
-       * only if the pad is resetted 
+       * only if the pad is resetted
        */
       GstClockTime collected_duration = GST_CLOCK_TIME_NONE;