What we do and don't expose to client toolkits

[Alan Griffiths]

When clients toolkits provide hints to place child surfaces using the
existing functions:

    mir_surface_spec_attach_to_foreign_parent();
    mir_connection_create_spec_for_tip();
    mir_connection_create_spec_for_menu(); or the proposed,
    mir_surface_spec_set_placement()

the toolkit wants to know where the child surface actually ends up in
order to render appropriately.

We currently have a policy not to provide any location information to
clients, so I want to be sure that I don't propose anything controversial.

In discussion with Chris he suggests that sending a
PlacementRelativeToParent{dx, dy} message is the right solution.

Doing this opens up an opportunity for clients to:

    1. probe the display boundaries using dummy placement requests. (The
    point is to provide the location before they render.)

    2. parent (and place) everything they do on a fullscreen surface
    (which they can conceal from the user). It does limit them to
    surface types that can be parented.

Thoughts and suggestions for the right way forward please.

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