webgl: add build.sh to build the transcoder/encoder with Emscripten

Convenience script (place: webgl/build.sh) that runs emcmake/cmake/make
in webgl/transcoder/build and webgl/encoder/build. Stops on the first
error (set -euo pipefail), builds both by default or a named target
(transcoder/encoder), supports --clean and -D... cmake pass-through, is
runnable from any directory, and auto-sources emsdk if emcmake is not on
PATH.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

webgl/build.sh

diff --git a/webgl/build.sh b/webgl/build.sh
new file mode 100755
index 0000000..79eefbf
--- /dev/null
+++ b/webgl/build.sh
@@ -0,0 +1,75 @@
+#!/usr/bin/env bash
+#
+# Build the WebAssembly transcoder and/or encoder with Emscripten.
+# Runs `emcmake cmake ../ && make` in each project's build/ dir.
+# Stops on the first error.
+#
+# Usage:
+#   ./build.sh                      # build both (transcoder, then encoder)
+#   ./build.sh transcoder           # build only the transcoder
+#   ./build.sh encoder              # build only the encoder
+#   ./build.sh --clean              # wipe each build dir's CMake cache first
+#   ./build.sh encoder -DKTX2_ZSTANDARD=OFF -DCMAKE_BUILD_TYPE=Debug
+#
+# Any -D... argument is forwarded to cmake. Runnable from any directory.
+#
+set -euo pipefail
+
+# Absolute path to this script's directory (webgl/), so CWD doesn't matter.
+WEBGL_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+
+# ---- pretty output (color only on a terminal) ----
+if [ -t 1 ]; then B=$'\e[1m'; G=$'\e[32m'; R=$'\e[31m'; Y=$'\e[33m'; N=$'\e[0m'; else B=; G=; R=; Y=; N=; fi
+info() { printf '%s==>%s %s\n' "$B$G" "$N" "$*"; }
+die()  { printf '%serror:%s %s\n' "$B$R" "$N" "$*" >&2; exit 1; }
+
+# ---- parse args ----
+CLEAN=0
+TARGETS=()
+CMAKE_ARGS=()
+for a in "$@"; do
+  case "$a" in
+    -c|--clean)         CLEAN=1 ;;
+    transcoder|encoder) TARGETS+=("$a") ;;
+    -D*)                CMAKE_ARGS+=("$a") ;;
+    -h|--help)          sed -n '3,14p' "$0" | sed 's/^# \{0,1\}//'; exit 0 ;;
+    *)                  die "unknown argument: '$a' (try --help)" ;;
+  esac
+done
+# Default: build both. Transcoder first (quick), then encoder.
+[ "${#TARGETS[@]}" -eq 0 ] && TARGETS=(transcoder encoder)
+
+# ---- toolchain check (auto-source emsdk if present but not on PATH) ----
+if ! command -v emcmake >/dev/null 2>&1 && [ -n "${EMSDK:-}" ] && [ -f "$EMSDK/emsdk_env.sh" ]; then
+  info "emcmake not on PATH; sourcing \$EMSDK/emsdk_env.sh"
+  # shellcheck disable=SC1091
+  source "$EMSDK/emsdk_env.sh" >/dev/null 2>&1 || true
+fi
+command -v emcmake >/dev/null 2>&1 || die "emcmake not found. Activate emsdk first, e.g.: source /path/to/emsdk/emsdk_env.sh"
+command -v cmake   >/dev/null 2>&1 || die "cmake not found."
+
+JOBS="$(nproc 2>/dev/null || echo 4)"
+
+# ---- build one project ----
+build_one() {
+  local name="$1" dir="$WEBGL_DIR/$1/build"
+  [ -f "$WEBGL_DIR/$1/CMakeLists.txt" ] || die "no CMakeLists.txt in webgl/$1"
+  info "Building $name -> $1/build  (make -j$JOBS)"
+  mkdir -p "$dir"
+  if [ "$CLEAN" -eq 1 ]; then
+    info "  cleaning CMake cache"
+    rm -rf "$dir/CMakeCache.txt" "$dir/CMakeFiles"
+  fi
+  ( cd "$dir" && emcmake cmake ../ "${CMAKE_ARGS[@]}" && make -j"$JOBS" )
+}
+
+# ---- go ----
+for t in "${TARGETS[@]}"; do
+  build_one "$t"
+done
+
+# ---- summary ----
+info "Success. Built artifacts:"
+for t in "${TARGETS[@]}"; do
+  find "$WEBGL_DIR/$t/build" -maxdepth 1 -name '*.wasm' -printf '    %p (%s bytes)\n' 2>/dev/null | sort
+done