use original image type (8-bit grayscale or 24-bit RGB) in the case of RAW video (ticket #782)

modules/highgui/src/cap_ffmpeg.cpp

@@ -1270,6 +1270,8 @@ bool CvVideoWriter_FFMPEG::open( const char * filename, int fourcc,
       bitrate_scale = 128;
       break;
     case CODEC_ID_RAWVIDEO:
+      codec_pix_fmt = input_pix_fmt;
+      break;
     default:
         // good for lossy formats, MPEG, etc.
         codec_pix_fmt = PIX_FMT_YUV420P;