vidcodec: add support for encode- or decode-only vidcodec modules
gst_video: new vidcodec module using gstreamer

the original code was based on avcodec, and written by
Victor Sergienko and Fadeev Alexander. Later the code was
reviewed by Alfred and cleaned up a bit.

this module can use hardware-encoding of H.264 for
devices that support it (via gstreamer), by tweaking
the pipeline string a bit.