In the vt_h264_video_info function, the format of the video to be encoded is
always being set to VIDEO_FORMAT_NV12, dispite the presence of code to set
the video format to VIDEO_FORMAT_I420 or VIDEO_FORMAT_I444. This commit fixes
that function to respect the video format choice of the user.

In addition, whilst testing this fix initially, I also discovered that the
4:4:4 colour format is not supported by the VideoToolbox H264 encoder.
Looking at the VideoToolbox code in ffmpeg as a reference, the ffmpeg code
errors out if a color format other than NV12 or I420 is set. Therefore, this
commit also logs a warning about I444 not being supported, and uses the NV12
default.