icons not honoured with gnome-shell on wayland

[Antoine Martin]

I'm sure this happened before.

Since r12369, we can save the icons using XPRA_SAVE_WINDOW_ICONS=1 and to be safe, disable the icon overlay with XPRA_ICON_OVERLAY=0.
Sure enough, the icon saved is the correct one.

I don't think that #2727 is relevant here: it should already be fixed, and wouldn't affect just wayland.

Maybe related to r15826?

It looks to me like wayland decides to use a single icon for all the windows we create, no matter what icon we specify for them.
Maybe we can somehow tell it not to do that?
Using ​Gtk.Application perhaps? ​gtk-3-tutorial : application

[migration script]

this ticket has been moved to: ​https://github.com/Xpra-org/xpra/issues/2988