glimagesink: get_caps should return caps with current size

Submitted by Philippe Renon

This would help upstream elements that want to produce video buffers of the same size as the sink.

It would also help when renegotiating after the sink surface is resized.

The Upstream caps (re)negotiation documentation (http://gstreamer.freedesktop.org/data/doc/gstreamer/head/pwg/html/section-nego-upstream.html) can be understood as requiring that the sink returns caps with the current size.

Other sinks, like ximagesrc, return caps with the current size.