Adds an GST_VIDEO_OVERLAY_COMPOSITION_META_API_TYPE query to glupload
and glimagesink. Detects the query from the downstream elements, so
it is executed only when downstream supports the overlay API.
This makes pipelines with textoverlay ! glupload ! gldownload ! xvimagesink possible.
Uses allocation meta struct for passing the window size upstream.
https://bugzilla.gnome.org/show_bug.cgi?id=745107
I notice that if you stop the pipeline during a renegotiation
the upload may be NULL while an allocation query is being run.
In that scenario, returning FALSE to the allocation query is the
best thing.
In some upload implementations the out buffer has more than one references,
turning the buffer not writable, so it won't be possible to modify its
meta-data.
This patch moves the meta-data copy before increasing the reference of the out
buffer.
https://bugzilla.gnome.org/show_bug.cgi?id=746173
