Fix some issues with shmsrc pipeline and video dimensions (bug #29)

Shmsrc seems to work with a gstreamer 1.4 build, but not with distro gstreamer
(segfault due to missing fix https://bugzilla.gnome.org/show_bug.cgi?id=731093)
Reader / writer of live source need to be using same gstreamer build, otherwise
some pipeline problems appear in deserializing and caps detection.

Playback of the first shape mapped with the live source doesn't seem to work (?),
so this bug stiil needs investigation.