Add ffmpeg to Nix dependencies

nix/dependencies.nix

@@ -1,6 +1,7 @@
 { pkgs ? import <nixpkgs> {
     overlays = [
       (import ./overlay/replace-torch.nix { })
+      (import ./overlay/ffmpeg.nix)
     ];
     config = {
       allowUnfree = true;

nix/overlay/ffmpeg.nix

@@ -0,0 +1,35 @@
+# https://github.com/NixOS/nixpkgs/blob/master/pkgs/development/libraries/ffmpeg/generic.nix
+self: super: {
+  ffmpeg = super.ffmpeg.override {
+    withDebug = false;
+    buildFfplay = false;
+    withHeadlessDeps = true;
+    withCuda = true;
+    withNvdec = true;
+    withFontconfig = true;
+    withGPL = true;
+    withAom = true;
+    withAss = true;
+    withBluray = true;
+    withFdkAac =true;
+    withFreetype = true;
+    withMp3lame = true;
+    withOpencoreAmrnb = true;
+    withOpenjpeg = true;
+    withOpus = true;
+    withSrt = true;
+    withTheora = true;
+    withVidStab = true;
+    withVorbis = true;
+    withVpx = true;
+    withWebp = true;
+    withX264 = true;
+    withX265 = true;
+    withXvid = true;
+    withZmq = true;
+    withUnfree = true;
+    withNvenc = true;
+    buildPostproc = true;
+    withSmallDeps = true;
+  };
+}

nix/shell.nix

@@ -11,6 +11,7 @@ pkgs.mkShell {
     pkgs.curl
     pkgs.linuxPackages.nvidia_x11
     pkgs.ncurses5
+    pkgs.ffmpeg
   ];
   shellHook = ''
     export CUDA_PATH=${dependencies.cudatoolkit}