This allows us to use the GstVideoOverlayComposition API and correctly
handle pre-multiplied alpha, while also only doing the alpha conversion
once instead of twice for the whole frame.
At a later point we can attach the meta to the buffer instead of
blending ourselves if downstream supports that.
https://bugzilla.gnome.org/show_bug.cgi?id=797091
Allows applications to connect to the "draw" signal of
the element and do their custom drawing there.
Includes an example application demonstrating usage.
Fixes: https://bugzilla.gnome.org/show_bug.cgi?id=595520