nlesource: should implement uridecodebin's autoplug-query according with nleoperation's caps
This is related with issue #131
In order to negotiate specific caps features, for example, nleoperation uses glvideomixer, it could accept memory:DMABuf from decoders in nlesource, nlesource should implement autoplug-query signal from uridecodebin and reply with the caps supported by nleoperation sink caps.
Another task could be to support uridecodebin3 in nlesource.