Build ffmpeg-mpv, shaderc and crossc from source, since they are not
packaged in MSYS2. Also, add some more explicit --enable flags to the
mpv build to make sure things like D3D11, D3D11VA hwaccels and Vulkan
are auto-detected.
AppVeyor build workers have Git for Windows on the %PATH%, which
includes some MSYS2 binaries. Having multiple MSYS2 installations on the
%PATH% causes cygheap errors, so remove them and make sure %PATH% only
has stuff we need.
