Fix string drawing truncation in combo boxes

This fixes FWNX-861, which reports the text showing in the combo box as
being truncated at word boundaries, leading to misleading displays.  It
also fixes Xamarin 10321, where the misleading truncation in the test
program occurs in the dropdown instead of the textbox.  (I don't know why
the bug manifests differently between the simple test program and Flex.)

Change-Id: I12b346e71ebb8382a661dd0abae9598cd4c58140