* cache window size event and update handle ratio
* init width with 1, don't use 0
* don't update overlay when receiving same window size
* receive window size from allocation query
https://bugzilla.gnome.org/show_bug.cgi?id=751157
This makes pipelines with multiple textoverlay elements possible.
The meta data is collected from the upstream textoverlay element,
merged into a new GstVideoOverlayComposition and passed down downstream.
https://bugzilla.gnome.org/show_bug.cgi?id=751157
This prevents the ugly hack where the text_sink pad template
was only added for textoverlay but not for the subclasses.
Also makes this work with the core change that made
subclasses inherit the templates of their parent class.
