Input method: update input method popup positions on text input commit

[lilydjwg]

A text input commit may come with a set_cursor_rectangle request, and we need to update the input method popup to follow the text cursor on the client side.

Previously we run the update on popup surface commit, but events may come out of order, i.e. the input method draws a new surface buffer as soon as it commits the input string, and then the client window will take some time to process the committed string and figure out the new position of cursor.