textoverlay: support forced updating text when new text buffer comes in
Submitted by cjunwang
some text producer element, such as closed caption decoder "cea708dec", output text buffer whose duration is not fixed. That means when text buffer display, it's not determined yet when text will be destroyed. https://bugzilla.gnome.org/show_bug.cgi?id=704881
To support this kind of instant buffer, added a property "force-update-text", to allow update text when new text buffer come and existed buffer duration is not finished. It is extremely useful to show scrollable closed caption text.
This property's default value is false. so it would not influence current logics.