cairooverlay: Add property for drawing on a transparent surface and then blending

cairooverlay: Add property for drawing on a transparent surface and then blending

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