build: lower required FFmpeg version

The FFmpeg version was last bumped a long time ago, except in commit
1638fa7b46, where it was used for some obscure pixel
format.

This is pretty annoying, so make it optional.

video/fmt-conversion.c

@@ -35,7 +35,9 @@ static const struct {
     {IMGFMT_RGB24, AV_PIX_FMT_RGB24},
     {IMGFMT_UYVY,  AV_PIX_FMT_UYVY422},
     {IMGFMT_NV12,  AV_PIX_FMT_NV12},
+#if LIBAVUTIL_VERSION_MICRO >= 100 && LIBAVUTIL_VERSION_INT >= AV_VERSION_INT(56, 27, 0)
     {IMGFMT_NV24,  AV_PIX_FMT_NV24},
+#endif
     {IMGFMT_Y8,    AV_PIX_FMT_GRAY8},
     {IMGFMT_Y16, AV_PIX_FMT_GRAY16},
     {IMGFMT_420P,  AV_PIX_FMT_YUV420P},

wscript

@@ -424,7 +424,7 @@ iconv support use --disable-iconv.",
 ]
 
 ffmpeg_pkg_config_checks = [
-    'libavutil',     '>= 56.27.100',
+    'libavutil',     '>= 56.12.100',
     'libavcodec',    '>= 58.16.100',
     'libavformat',   '>= 58.9.100',
     'libswscale',    '>= 5.0.101',