tests/fate/dnxhd: add dnxhr prefix tests

The data offset value in the prefix increases as the heights increases.
This test adds a tests for some common dnxhr heights.

can somebody add the following test footage to fate/dnxhd?
https://dl.dropboxusercontent.com/u/170952/fate/dnxhd/prefix-256x1536.dnxhr
https://dl.dropboxusercontent.com/u/170952/fate/dnxhd/prefix-256x1716.dnxhr
https://dl.dropboxusercontent.com/u/170952/fate/dnxhd/prefix-256x2048.dnxhr
https://dl.dropboxusercontent.com/u/170952/fate/dnxhd/prefix-256x2160.dnxhr
https://dl.dropboxusercontent.com/u/170952/fate/dnxhd/prefix-256x3212.dnxhrSigned-off-by: Michael Niedermayer <michael@niedermayer.cc>

tests/fate/dnxhd.mak

 FATE_DNXHD = fate-dnxhd-mbaff     \
              fate-dnxhr-444       \
-             fate-dnxhr-parse
+             fate-dnxhr-parse     \
+             fate-dnxhr-prefix1   \
+             fate-dnxhr-prefix2   \
+             fate-dnxhr-prefix3   \
+             fate-dnxhr-prefix4   \
+             fate-dnxhr-prefix5
 
 FATE_SAMPLES_AVCONV-$(call DEMDEC, MOV, DNXHD) += $(FATE_DNXHD)
 fate-dnxhd: $(FATE_DNXHD) $(FATE_VCODEC_DNXHD)
@@ -8,3 +13,8 @@ fate-dnxhd: $(FATE_DNXHD) $(FATE_VCODEC_DNXHD)
 fate-dnxhd-mbaff: CMD = framecrc -flags +bitexact -idct simple -i $(TARGET_SAMPLES)/dnxhd/dnxhd100_cid1260.mov -pix_fmt yuv422p10le
 fate-dnxhr-444:   CMD = framecrc -flags +bitexact -idct simple -i $(TARGET_SAMPLES)/dnxhd/dnxhr444_cid1270.mov -pix_fmt yuv444p10le
 fate-dnxhr-parse: CMD = framecrc -flags +bitexact -idct simple -i $(TARGET_SAMPLES)/dnxhd/dnxhr_cid1274.dnxhr -pix_fmt yuv422p
+fate-dnxhr-prefix1: CMD = framecrc -flags +bitexact -idct simple -i $(TARGET_SAMPLES)/dnxhd/prefix-256x1536.dnxhr -pix_fmt yuv422p
+fate-dnxhr-prefix2: CMD = framecrc -flags +bitexact -idct simple -i $(TARGET_SAMPLES)/dnxhd/prefix-256x1716.dnxhr -pix_fmt yuv422p
+fate-dnxhr-prefix3: CMD = framecrc -flags +bitexact -idct simple -i $(TARGET_SAMPLES)/dnxhd/prefix-256x2048.dnxhr -pix_fmt yuv422p
+fate-dnxhr-prefix4: CMD = framecrc -flags +bitexact -idct simple -i $(TARGET_SAMPLES)/dnxhd/prefix-256x2160.dnxhr -pix_fmt yuv422p
+fate-dnxhr-prefix5: CMD = framecrc -flags +bitexact -idct simple -i $(TARGET_SAMPLES)/dnxhd/prefix-256x3212.dnxhr -pix_fmt yuv422p

tests/ref/fate/dnxhr-prefix1

+#tb 0: 1/25
+#media_type 0: video
+#codec_id 0: rawvideo
+#dimensions 0: 256x1536
+#sar 0: 0/1
+0,          0,          0,        1,   786432, 0x5a07f1fe
+0,          1,          1,        1,   786432, 0x5b5acaed
+0,          2,          2,        1,   786432, 0x1a9aacb5
+0,          3,          3,        1,   786432, 0x12ddf22d
+0,          4,          4,        1,   786432, 0xde2515f8

tests/ref/fate/dnxhr-prefix2

+#tb 0: 1/25
+#media_type 0: video
+#codec_id 0: rawvideo
+#dimensions 0: 256x1716
+#sar 0: 0/1
+0,          0,          0,        1,   878592, 0x9a99e1db
+0,          1,          1,        1,   878592, 0x9317b608
+0,          2,          2,        1,   878592, 0x2d6c9b2f
+0,          3,          3,        1,   878592, 0xa7f0e3fe
+0,          4,          4,        1,   878592, 0x98330125

tests/ref/fate/dnxhr-prefix3

+#tb 0: 1/25
+#media_type 0: video
+#codec_id 0: rawvideo
+#dimensions 0: 256x2048
+#sar 0: 0/1
+0,          0,          0,        1,  1048576, 0xa2eb0c23
+0,          1,          1,        1,  1048576, 0x2149e4f9
+0,          2,          2,        1,  1048576, 0xbdfacc9b
+0,          3,          3,        1,  1048576, 0x64040c52
+0,          4,          4,        1,  1048576, 0x9fec31b4

tests/ref/fate/dnxhr-prefix4

+#tb 0: 1/25
+#media_type 0: video
+#codec_id 0: rawvideo
+#dimensions 0: 256x2160
+#sar 0: 0/1
+0,          0,          0,        1,  1105920, 0xcc1cc322
+0,          1,          1,        1,  1105920, 0xd3038a1d
+0,          2,          2,        1,  1105920, 0xc0717a5a
+0,          3,          3,        1,  1105920, 0xed6fc31f
+0,          4,          4,        1,  1105920, 0xdcb8d6c4

tests/ref/fate/dnxhr-prefix5

+#tb 0: 1/25
+#media_type 0: video
+#codec_id 0: rawvideo
+#dimensions 0: 256x3212
+#sar 0: 0/1
+0,          0,          0,        1,  1644544, 0xcea2b72d
+0,          1,          1,        1,  1644544, 0xcd80819d
+0,          2,          2,        1,  1644544, 0x4a27745e
+0,          3,          3,        1,  1644544, 0xdf82b72c
+0,          4,          4,        1,  1644544, 0xdf87cb6e