avilib: use  integer size matching the format specifier
window_geometry_saver: avoid sign difference comparison warning
attachment target: avoid compiler warning about possible use of uninitialized
os.h: don't #define WINNT version macros
configure: look for Boost libs in multiarch directories
mkvmerge: check result of tag parser file before using it

The result is a null smartptr if there are no tags inside the file. So
don't dereference it.

Fixes #921.
tests: intentional update due to f540dfa
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.