Source: switch libebml/libmatroska submodules to new URL step 1
Source: switch libebml/libmatroska submodules to new URL step 2
Refactoring: avcc_c class for packing/unpacking AvcC structures
MP4 reader: implement VobSub support

Implements #815.
Source: avoid warnings: uninitialized vars/invalid lambda default args
Translations: update Spanish