wayland: avoid 0 width/height anchor rectangle

wayland: avoid 0 width/height anchor rectangle

Passing a rectangle with zero width or height to xdg_shell-v6
set_anchor_rect() will cause a protocol error and terminate the client,
as with gedit when pressing the Win key.

Reason for this is because the rectangle used to set the anchor comes
from gtk_text_layout_get_iter_location() which uses the pango layout
width/height, which can be empty if there is not character at the given
location.

Make sure we don't use 0 as width or height as an anchor rectangle to
avoid the protocol error, and compensate the logical position of the
given rectangle if the size is changed, so that the actual position
remains as expected by the client.

https://bugzilla.gnome.org/show_bug.cgi?id=777176