The hardware decoder can become (temporarily) unavailable across
VTDecompressionSessionCreate/Destroy calls. During negotiation if the currently
configured caps are still accepted by downstream we keep using them so we don't
have to destroy and recreate the decoding session.
This indirectly fixes https://bugzilla.gnome.org/show_bug.cgi?id=767429, by
making vtdec stick to GLMemory.