[FFmpeg-devel] Improve Matroska tag name prefix

[Pierre Choffet]

The current implementation of the Matroska tags reader prefixes tag names with TargetType[1] only. In files 
with Tag having no TargetType element, all tags which share the same name but apply at different levels overwrite 
each other.

This patch adds the TargetTypeValue[1] in the prefix. This element is not mandatory either, but it has a 
default value which allows to strictly separate tags by level when it's not here.

[1]: http://matroska.org/technical/specs/tagging/index.html