convertframe: Fix trivial memory leak in debug statement

gst_element_get_name() requires the caller to g_free() the return value

https://bugzilla.gnome.org/show_bug.cgi?id=719850

convertframe: Fix indention
videodecoder: make _release_frame external API

... so subclasses can release a frame all the way (also from frame list)
without having to pass through _finish_frame or _drop_frame.
The latter may not be applicable, or may or may not have already
been called for the frame in question.

See https://bugzilla.gnome.org/show_bug.cgi?id=693772
videodecoder: fix spelling error in debug message
videodecoder: add new API to docs and defs
videodecoder: gst_video_decoder_release_frame() is available since 1.2.2
vorbistag: Read image-type from the GstSample info struct

But for backwards compatibility keep reading it from the caps and only
use the info struct if the caps don't contain the image-type.