gtk3 public API requests

[Christian Hergert <christian hergert me>]

Hey everyone,

I'm lobbying to make 2 functions public API in the stable branch (3.x)
that would help me be able do some fancy things in Builder/Dazzle (which
I can make work fine on Xorg already).

If there are objections, I'd like to hear from people so I can find a
way to make forward progress.

The functions I'd like to see made public:

  - gtk_window_set_use_subsurface()
  - gdk_window_move_to_rect()

The Gdk function is currently accessible if you're compiling Gtk since
it can access the private API vtable. Widgets like GtkMenu use this to
do positioning. It didn't make it to public API in time for 3.22, but
had we continued to do a 3.24, it seems likely that would have happened.

The gtk_window_set_use_subsurface() is basically only useful when on
Wayland, but it helps ensure that we get a subsurface for transient
popup windows. I'm open to other ways to solve the problem, like
GDK_WINDOW_SUBSURFACE, if they allow me to ensure that my window is
backed by a subsurface on Wayland.

Again, this API is available internally to Gtk so widgets like ComboBox
can use it, but if you're not a GtkWidget, you're outta luck.

Thoughts?

-- Christian