nix: rename custom -> packages

nix/packages/ffgun.nix

@@ -0,0 +1,42 @@
+# adapted from https://github.com/neXromancers/hacksaw
+
+{
+  writeShellApplication,
+  shotgun,
+  ffmpeg_5-full,
+}:
+
+writeShellApplication {
+  name = "ffgun";
+  runtimeInputs = [
+    shotgun
+    ffmpeg_5-full
+  ];
+  text = ''
+    dir=/tmp
+    current=$(date +%F_%H-%M-%S)
+
+    echo "$dir/$current.mp4"
+
+    hacksaw -n | {
+        IFS=+x read -r w h x y
+
+        w=$((w + w % 2))
+        h=$((h + h % 2))
+
+        ffmpeg               \
+            -v 16            \
+            -r 30            \
+            -f x11grab       \
+            -s "''${w}x''${h}"   \
+            -i ":0.0+$x,$y"  \
+            -preset slow     \
+            -c:v h264        \
+            -pix_fmt yuv420p \
+            -crf 20          \
+            "$dir/$current.mp4"
+    }
+  '';
+
+  meta.description = "Shotgun's integration script with ffmpeg from their github";
+}