WebM has a couple of specific requirements we need to handle.
Idea is to set this flag once and just rely on mux->is_webm
at run time instead of repeatedly figuring this out from
GST_MATROSKA_DOCTYPE_WEBM (which requires a strcmp()).
Support for TAG_IMAGE and TAG_ATTACHMENT is commented out; this requires caps
on buffers which is gone from 0.11.
Segment handling in the demuxer is a bit complex; I added some FIXME comments
in places where I'm not yet sure if I ported correctly.
