avformat: add vapoursynth wrapper

This can "demux" .vpy files. Autodetection of .vpy scripts is
intentionally not done, because it would be a major security issue. You
need to force the format, for example with "-f vapoursynth" for the
FFmpeg CLI tools.

Some minor code copied from other LGPL parts of FFmpeg.

I did not find a good way to test a few of the more obscure VS features,
like VFR nodes, compat pixel formats, or nodes with dynamic size/format
changes. These can be easily implemented on demand.

configure

@@ -303,6 +303,7 @@ External library support:
   --disable-sdl2           disable sdl2 [autodetect]
   --disable-securetransport disable Secure Transport, needed for TLS support
                            on OSX if openssl and gnutls are not used [autodetect]
+  --enable-vapoursynth     enable VapourSynth demuxer [no]
   --disable-xlib           disable xlib [autodetect]
   --disable-zlib           disable zlib [autodetect]
 
@@ -1724,6 +1725,7 @@ EXTERNAL_LIBRARY_LIST="
     mediacodec
     openal
     opengl
+    vapoursynth
 "
 
 HWACCEL_AUTODETECT_LIBRARY_LIST="
@@ -3091,6 +3093,7 @@ libx265_encoder_deps="libx265"
 libxavs_encoder_deps="libxavs"
 libxvid_encoder_deps="libxvid"
 libzvbi_teletext_decoder_deps="libzvbi"
+vapoursynth_demuxer_deps="vapoursynth"
 videotoolbox_suggest="coreservices"
 videotoolbox_deps="corefoundation coremedia corevideo"
 videotoolbox_encoder_deps="videotoolbox VTCompressionSessionPrepareToEncodeFrames"
@@ -6133,6 +6136,8 @@ enabled rkmpp             && { require_pkg_config rkmpp rockchip_mpp  rockchip/r
                                { enabled libdrm ||
                                  die "ERROR: rkmpp requires --enable-libdrm"; }
                              }
+enabled vapoursynth       && require_pkg_config vapoursynth "vapoursynth-script >= 42" VSScript.h vsscript_init
+
 
 if enabled gcrypt; then
     GCRYPT_CONFIG="${cross_prefix}libgcrypt-config"