Make BufferView::singeParUpdate more robust

Make BufferView::singeParUpdate more robust

In some cases, it might happen that this method is called with no
metrics or position known for the current paragraph.

Take care of these cases to avoid assertions.

Remove setting of inset positions in the method, but make sure that
updateMetrics(false) is always called to get everything right.

In the new code, updateMetrics(bool) is the method that sets
everything right with minimal effort.

(cherry picked from commit 89ab9eb569ec0eea87d9a8c269eb87507934e1c5)