ClutterInputMethods content hint or purpose will be set again (even to the
same values) whenever a wayland client sends us a new content type/purpose.
Gtk appears to always set a content purpose on wl_text_input changes, so we
currently set and notify the "content-purpose" property on every change in a
gtk text field.
Since the OSK is gnome-shell listens to this property and re-generates its
entire layout when the content-purpose prop gets notified, this is currently
causing lag/freezes on every keypress in the OSK in gnome-shell.
So ensure to not notify these properties in case they're equal and set the
properties in the same way as we usually set them instead of going via
GObject.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3645>
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