qtmux/mp4mux can't change size after first buffer.

Submitted by Marcin Kolny @loganek

Link to original bug (#747184)

Description

Created attachment 300752
test application

Hi,
I've udp video stream, which can change its resolution. I've noted, that qtmux/mp4mux fails, when video size is changed. I did simple application, which triggers this bug (see attachment).
I digged a little, and noted, that the problem is in method gst_qt_mux_video_sink_set_caps. At the beginning, function checks, whether previous caps are subset of new caps (gst_qtmux_caps_is_subset_full). It obviously isn't true, because caps have other size. I saw comment above - Does it mean, that there is no possibility to renegotiate caps in mp4mux/qtmux?
IMO it's a bug in a plugin, but have no idea how to fix it in a proper way.

Attachment 300752, "test application":
muxtest.c