Add Avisynth video encoding workflow

tools/avisynth_runner/build_windows.sh

@@ -0,0 +1,38 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+script_dir="$(cd -- "$(dirname -- "${BASH_SOURCE[0]}")" && pwd)"
+include_dir="${AVISYNTH_INCLUDE_DIR:-}"
+
+if [[ -z "$include_dir" ]]; then
+  for candidate in \
+    /tmp/media_batch_avisynth/avisynthplus/avs_core/include \
+    /tmp/media_batch_avisynth/install/include/avisynth
+  do
+    if [[ -f "$candidate/avisynth_c.h" ]]; then
+      include_dir="$candidate"
+      break
+    fi
+  done
+fi
+
+if [[ -z "$include_dir" || ! -f "$include_dir/avisynth_c.h" ]]; then
+  echo "Could not find avisynth_c.h." >&2
+  echo "Set AVISYNTH_INCLUDE_DIR to an AviSynth+ include directory." >&2
+  exit 1
+fi
+
+build_one() {
+  local cc="$1"
+  local output="$2"
+  if ! command -v "$cc" >/dev/null 2>&1; then
+    echo "Skipping $output; $cc not found." >&2
+    return 0
+  fi
+  "$cc" -I"$include_dir" -O2 -Wall -Wextra \
+    -o "$script_dir/$output" "$script_dir/mbt_avs_runner.c"
+  echo "built $script_dir/$output"
+}
+
+build_one "${WIN64_CC:-x86_64-w64-mingw32-gcc}" mbt_avs_runner.exe
+build_one "${WIN32_CC:-i686-w64-mingw32-gcc}" mbt_avs_runner32.exe