Make 32bit RGB dshow input opaque.

libavdevice/dshow.c

@@ -89,7 +89,7 @@ static enum AVPixelFormat dshow_pixfmt(DWORD biCompression, WORD biBitCount)
             case 24:
                 return AV_PIX_FMT_BGR24;
             case 32:
-                return AV_PIX_FMT_RGB32;
+                return AV_PIX_FMT_0RGB32;
         }
     }
     return avpriv_find_pix_fmt(ff_raw_pix_fmt_tags, biCompression); // all others