Support biCompression I420

Originally committed as revision 12821 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavdevice/vfwcap.c

@@ -96,6 +96,8 @@ static enum PixelFormat vfw_pixfmt(DWORD biCompression, WORD biBitCount)
     switch(biCompression) {
     case MKTAG('Y', 'U', 'Y', '2'):
         return PIX_FMT_YUYV422;
+    case MKTAG('I', '4', '2', '0'):
+        return PIX_FMT_YUV420P;
     case BI_RGB:
         switch(biBitCount) { /* 1-8 are untested */
             case 1: