nvenc: Add support for more encoding option

nvenc: Add properties to support bframe encoding if device supports it

Note that bframe encoding capability varies with GPU architecture

nvenc: Refactoring internal buffer pool structure

To support rc-lookahead and bframe encoding, nvenc needs one more
staging queue, because NvEncEncodePicture can return NV_ENC_ERR_NEED_MORE_INPUT
but which was not considered so far.
As documented by NVENC programming guide, pending buffers should wait
other inputs until NvEncEncodePicture returns success.

New encoding flow is
- Submit raw picture buffer to encoder with NvEncEncodePicture
- The submitted input/output buffer pair will be queued to pending_queue
  - If NvEncEncodePicture returned success, then move all pair in pending_queue to final stage
  - Otherwise, wait more input raw pictures.

Another change is dropping NV_ENC_LOCK_INPUT_BUFFER usage.
So now nvenc always uses CUDA memory input buffer. As a result,
both opengl and system memory handling are unified.

nvenc: Remove pointless iteration and cleanup some code

* The number of iteration is always one so the iteration is useless
and that makes code complicated.
* Also defining named structure can make code more readable.
* g_free is null safe

nvenc: Add more rate-control options

New rate-control modes are introduced (if device can support)
* cbr-ld-hr: CBR low-delay high quality
* cbr-hq: CBR high quality
* vbr-hq: VBR high quality

Also, various configurable rate-control related properties are added.

nvenc: Add support for weighted prediction option

If device can support the weighted prediction option, the allowed
maximum value of "weighted-pred" property will be set to 1.
Users can inspect the availability of this option via GParamSpec

nvenc: Add property for AUD insertion

Make AUD insertion configurable option