videotoolbox: don't let it handle crop

1. Videotoolbox doesn't handle codec info. It do crop the right
and bottom side if codec_size > output_size
2. There is no API to configure i_x_offset/i_y_offset to videotoolbox
3. It was a bug to let videotoolbox to crop and don't reset
i_x_offset/i_y_offset.