Fails to notice input purpose changes

When a text field changes its input purpose but nothing else changes squeekboard fails to do the layout change although there's a content_type message being sent:

[3615945,485] zwp_input_method_v2@28.surrounding_text("", 0, 0)
[3615945,633] zwp_input_method_v2@28.text_change_cause(0)
[3615945,677] zwp_input_method_v2@28.content_type(0, 13)
[3615945,721] zwp_input_method_v2@28.done()

This can easily be tested by using a GtkTextEntry that changes it's input-purpose randomly. The upcoming MR will have a test case.