plugins: add mpeg2 encoder element.

plugins: add mpeg2 encoder element.

Add GstVaapiEncodeMPEG2 element object. The actual plug-in element
is called "vaapiencode_mpeg2".

Valid properties:
- rate-control: rate control mode (default: cqp - constant QP)
- bitrate: desired bitrate in kbps (default: auto-calculated)
- key-period: maximal distance between two key frames (default: 30)
- max-bframes: number of B-frames between I and P (default: 2)
- quantizer: constant quantizer (default: 8)

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