qsv: Various enhancement

    qsvenc: Simplify property update helpers

    Takes lock outside of helper methods, and protect property getter too

    qsvh264enc: Add more encoding options

    Adding trellis quantization, frame size, slice size and number of
    slices related encoding options

    qsv: Use std::once_flag

    GLib's once implementation will take mutex every time
    in case of non-GCC build

    qsvh264enc: Optimization for byte-stream to packetized format conversion

    Allocate single memory instead of per NAL