codec: videotoolbox: fix incorrect format display size

The input sample's CMVideoFormatDescription needs to be created with visible pixels, not the buffer dimensions. Not doing that causes the output image buffer to report incorrect visible size when queried with CVImageBufferGetDisplaySize.

This can cause issues on display/encoding paths that solely rely on CVBuffer metadata, like AVSampleBufferDisplayLayer or VideoToolbox CVPX encoding.

Fix #28703 (closed)