mov: Check angle rather than full matrix when updating SAR

When the display matrix is not the identity one, but the rotation angle
is zero, there is no need to update the sample aspect ratio.

Otherwise, it is possible to obtain negative values which interferes
with transcoding in later stages. This kind of behaviour is reproducible
on mov files with "major_brand: MSNV".

CC: libav-stable@libav.org
Signed-off-by: Vittorio Giovara <vittorio.giovara@gmail.com>

libavformat/mov.c

@@ -35,6 +35,7 @@
 #include "libavutil/time_internal.h"
 #include "libavutil/avstring.h"
 #include "libavutil/dict.h"
+#include "libavutil/display.h"
 #include "libavutil/opt.h"
 #include "libavcodec/ac3tab.h"
 #include "avformat.h"
@@ -2578,15 +2579,10 @@ static int mov_read_tkhd(MOVContext *c, AVIOContext *pb, MOVAtom atom)
     }
 
     // transform the display width/height according to the matrix
-    // skip this if the display matrix is the default identity matrix
-    // or if it is rotating the picture, ex iPhone 3GS
+    // skip this if the rotation angle is 0 degrees
     // to keep the same scale, use [width height 1<<16]
-    if (width && height &&
-        ((display_matrix[0][0] != 65536  ||
-          display_matrix[1][1] != 65536) &&
-         !display_matrix[0][1] &&
-         !display_matrix[1][0] &&
-         !display_matrix[2][0] && !display_matrix[2][1])) {
+    if (width && height && sc->display_matrix &&
+        av_display_rotation_get(sc->display_matrix) != 0.0f) {
         for (i = 0; i < 2; i++)
             disp_transform[i] =
                 (int64_t)  width  * display_matrix[0][i] +