mmg: don't pass log messages to the default log target
build system: define WINVER and _WIN32_WINNT from the Rakefile
Update included libEBML and libMatroska to current HEADs
tests: move "srt" sub-dir
cosmetics
Translations: update list of translatables
mkvextract: don't allow the same ID multiple times

Implements #914.
tests: add test case for all five supported UTF encodings
common: refactor UTF reader code in order to avoid a compiler warning about access beyond array boundaries
cosmetics: indentation; use namespace for whole file
refactoring: rename vp8_video_packetizer_c to vpx_video_packetizer_c
mkvmerge: implement support for muxing VP9

Can be read from IVF and Matroska/WebM files. Implements #899.
mkvextract: implement support for extracting VP9

See #899.