Implement handling of row_flags for row breaking

Implement handling of row_flags for row breaking

To this end, add the helper function needsRowBreak which computes the
effect of two consecutive row flags. This function implements the
priorities described in RowFlags.h.

This function is called with the relevant flags, or NoBreak* when at
boundaries and updates need_new_row.

Some common code is factored in a new cleanupRow() helper.