swscale: fix bt709 yuv2rgb coefficients.

They were derived from incorrect values cr=0.2125 (should be cr=0.2126)
and cb=0.0721 (should be cb=0.0722).

libswscale/yuv2rgb.c

@@ -47,8 +47,8 @@
  * where Y = cr * R + cg * G + cb * B and cr + cg + cb = 1.
  */
 const int32_t ff_yuv2rgb_coeffs[8][4] = {
-    { 117504, 138453, 13954, 34903 }, /* no sequence_display_extension */
-    { 117504, 138453, 13954, 34903 }, /* ITU-R Rec. 709 (1990) */
+    { 117489, 138438, 13975, 34925 }, /* no sequence_display_extension */
+    { 117489, 138438, 13975, 34925 }, /* ITU-R Rec. 709 (1990) */
     { 104597, 132201, 25675, 53279 }, /* unspecified */
     { 104597, 132201, 25675, 53279 }, /* reserved */
     { 104448, 132798, 24759, 53109 }, /* FCC */