Implement GdkColorState support in gtk4paintablesink
(This is more intended as a tracker bug atm than as anything very actionable, but I'll update it once GTK has actionable APIs.)
This work is primarily happening in GTK's upstream sink and once it's done it should be relatively straightforward to port.
We are implementing colorimetry support in GTK and gtk4paintablesink should make use of it to forward the incoming GstVideoColorimetry by creating a matching GdkColorState to GdkTextures which will enable GTK to convert while rendering its UI in whatever colorspace it is operating, or doing straight graphics offloading to supported Wayland compositors.
The work to add cicp-based colorstates is currently happening in https://gitlab.gnome.org/GNOME/gtk/-/merge_requests/7453 (and in this comment I document the first time I successfully watched HDR content using GStreamer+GTK+Mutter last night.) I'll report back once that's landed.