Add a custom build task for getting the git version info

Eliminates the need to have a copy of git accessible in the msys
build environment and reduces the dependency on msys to just FFmpeg.

Probably trivially faster due to how slow shell scripts are on Windows.
Update ffmpeg
Add msbuild build system for FFmpeg

This eliminates the need for a msys environment to build any part of
Aegisub. Also happens to make building FFmpeg much faster (~3 minutes
vs. ~10 minutes for me).
Add more missing project dependencies for working parallel builds
Update readme
Fix test compilation on Windows