This fixes the "run" and "subprocess" commands on Windows, including
youtube-dl support.
Unix-like FD inheritance is emulated on Windows by using an undocumented
data structure[1] that gets passed to the newly created process in
STARTUPINFO.lpReserved2. It consists of two sparse arrays listing the
HANDLE and internal CRT flags corresponding to each FD. This structure
is used and understood primarily by MSVCRT, but there are other runtimes
and frameworks that can write it, like libuv.
The code for creating asynchronous "anonymous" pipes in Windows has been
enhanced and moved into windows_utils.c. This is mainly an artifact of
an unfinished future change to support anonymous IPC clients in Windows.
Right now, it's still only used in subprocess-win.c
[1]: https://www.catch22.net/tuts/undocumented-createprocess
This replaces the old backend that exclusively used EGL windowing with
one that can also use ANGLE's ability to render to directly to a
texture. The advantage of this is that it allows mpv to create the swap
chain itself and this allows mpv to use a flip-mode swap chain on a HWND
(which avoids problems with DirectComposition) and to use a longer swap
chain that has six backbuffers by default (which reportedly fixes
problems with rendering 24fps video on 24Hz monitors.)
Also, "screenshot window" should now work on DXGI 1.2 and up (Windows 8
and up.)
This is useful in particular for GetLastError, unfortunately, it's stil pretty
dumb with regards to WASAPI or D3D specific errors, so keep the
hresult_to_string switch.
Note that hresult_to_str() (coming from wasapi_explain_err()) is mostly
wasapi-specific, but since HRESULT error codes are unique, it can be
extended for any other use.
