[gpu-next] HDR metadata not forwarded in windowed mode

Important Information

Provide following Information:

• mpv version: mpv-dev-x86_64-v3-20220909-git-d433c5d
• Windows Version: Windows 10 21H2
• Source of the mpv binary: shinchiro winbuild repo
• Hardware: AMD RX580 / Nvidia RTX3050

Reproduction steps

I recently purchased a hdfury arcana to debug HDMI on my PC and other devices. I wanted to know if and which HDR metadata values mpv passes through to the TV. With mpv being in fullscreen, HDR turned on in Windows, and the following command line
mpv.com HDR.file -vo=gpu-next -hwdec=d3d11va -target-colorspace-hint=yes -v -fs
it works perfectly, as seen in the picture.

However when running mpv with the same command just without -fs and therefore in windowed mode, no metadata is passed through. The hdfury shows the default windows HDR values. (For nvidia all 0, for AMD 1499maxCLL/799maxFALL)
This behavior is quite unfortunate, as any software that embeds mpv and is not able to run it as fullscreen is out of luck passing through actual metadata. Also for daily use, where mpv should just run as a separate window it is unfortunate. It would be awesome if it could be added, either out of the box or with a dedicated allow command.

Expected behavior

Allow for metadata passthrough even in windowed mode.

Actual behavior

No metadata forwarded when running mpv in windowed mode.

Sample files

Any HDR file with metadata present

Start by reproducing the reported mpv command with -vo=gpu-next, -hwdec=d3d11va, -target-colorspace-hint=yes, and an HDR file, comparing fullscreen and windowed output. Trace the windowed-mode HDR metadata path and verify that the display receives the file's metadata rather than the default Windows HDR values.