Support for Ada and Objective-C was removed from MSYS2, which made
pacman refuse to update GCC while the gcc-ada and gcc-objc packages were
installed. Remove those packages before updating the others. Also remove
ANGLE, which has been removed from MSYS2, and add libplacebo, which is
now needed for the Vulkan VO.
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.
