clockoverlay: only rerender text if time string has changed

clockoverlay: only rerender text if time string has changed

The textoverlay element will rerender the text string whenever
overlay sets the 'need_render' flag to TRUE.  Previously, we
lazily set the flag to TRUE every time the time string was requested.
Now, we save a copy of the previously given string, and only set
'need_render' to TRUE if the string has changed.

In my tests with a 30fps video stream, and a time string including
a seconds field, this change reduced the CPU usage of the clockoverlay
element from 60% to 5%.

Fixes bug #627780.