dvdsubdec: fix incorrect colors.

On DVD and HD-DVD colors are stored in the order YCrCb (and not YCbCr) as mentioned in the specifications:
see DVD Specifications for Read-Only Disc / Part 3, 4.3 Program Chain Information (7) PGC_SP_PLT
see DVD Specifications for High Definition Disc, 5.2 Navigation for Standard Content (11) PGC_SDSP_PLT
see DVD Specifications for High Definition Disc, 5.2 Navigation for Standard Content (12) PGC_HDSP_PLT
see DVD Specifications for High Definition Disc, 5.5 Presentation Data (4) SET_COLOR2

When decoding a DVD or HD-DVD subtitle, the colors were incorrectly set.
Signed-off-by: Anton Khirnov <anton@khirnov.net>

libavcodec/dvdsubdec.c

@@ -34,8 +34,8 @@ static void yuv_a_to_rgba(const uint8_t *ycbcr, const uint8_t *alpha, uint32_t *
 
     for (i = num_values; i > 0; i--) {
         y = *ycbcr++;
-        cb = *ycbcr++;
         cr = *ycbcr++;
+        cb = *ycbcr++;
         YUV_TO_RGB1_CCIR(cb, cr);
         YUV_TO_RGB2_CCIR(r, g, b, y);
         *rgba++ = (*alpha++ << 24) | (r << 16) | (g << 8) | b;