plugins: add h264 encoder element.

plugins: add h264 encoder element.

Add GstVaapiEncodeH264 element object. The actual plug-in element
is called "vaapiencode_h264".

Valid properties:
- rate-control: rate control mode (default: none)
- bitrate: desired bitrate in kbps (default: auto-calculated)
- key-period: maximal distance between two key frames (default: 30)
- num-slices: number of slices per frame (default: 1)
- max-bframes: number of B-frames between I and P (default: 0)
- min-qp: minimal quantizer (default: 1)
- init-qp: initial quantizer (default: 26)

Signed-off-by: Gwenole Beauchesne <gwenole.beauchesne@intel.com>