The issue is that on X11, the window manager draws the title bar hence
uses the window-close button from the system theme, though on Wayland,
GTK draws its own decorations. Since we provide a window-close, we end
up with a disparity between this button and the minimize/maximize
buttons.
For now, let's just get rid of the window-close icon as an easy
solution, thus GIMP will always use the window-close icon from system
theme.
Maybe we can restore our own window-close icon later, but we should make
sure to at least have also window-maximize and window-minimize icons so
that the 3 buttons next to each others won't look too mismatched. And
probably we should also have smaller-size design because it doesn't look
so nice when huge (though it might simply have been the size mismatch
which gave this feeling).
68851c1aaa