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.
When the D3D11 backend was first written, SPIRV-Cross only had a C++ API
and no guarantee of API or ABI stability, so instead of using
SPIRV-Cross directly, mpv used an unofficial C wrapper called crossc.
Now that KhronosGroup/SPIRV-Cross#611 is resolved, SPIRV-Cross has an
official C API that can be used instead, so remove crossc and use
SPIRV-Cross directly.
Shaderc comes with a Python script that automatically fetches
"known-good" versions of its dependencies. Use that instead of manually
cloning dependencies to third-party.
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.
