message: ffmpeg: fix 'uchar' error
  > cap_ffmpeg_impl.hpp:1710:11: error: 'uchar' does not name a type
diff: @@ -1707,7 +1707,7 @@ bool CvVideoWriter_FFMPEG::writeFrame( const unsigned char* data, int step, int
  const int CV_STEP_ALIGNMENT = 32;
  const size_t CV_SIMD_SIZE = 32;
  const size_t CV_PAGE_MASK = ~(4096 - 1);
  - const uchar* dataend = data + ((size_t)height * step);
  + const unsigned char* dataend = data + ((size_t)height * step);
  if (step % CV_STEP_ALIGNMENT != 0 ||
  (((size_t)dataend - CV_SIMD_SIZE) & CV_PAGE_MASK) != (((size_t)dataend + CV_SIMD_SIZE) & CV_PAGE_MASK))
  {
  