Build with wasm exceptions so zimg errors don't abort the module

zimg throws and catches C++ exceptions for invalid parameters. Without
exception support the throw aborted the whole core (found by the fuzzer
with zscale=w=3:h=-1). -fwasm-exceptions also selects wasm setjmp/longjmp,
so every library is built with it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: lucasgelfond

Dockerfile

@@ -11,7 +11,9 @@ ARG EXTRA_LDFLAGS
 ARG MAKE_JOBS
 ENV INSTALL_DIR=/opt
 # FFmpeg's command line tools need threads since 6.1, so everything is built with pthreads.
-ENV CFLAGS="-I$INSTALL_DIR/include -pthread $EXTRA_CFLAGS"
+# zimg reports errors with C++ exceptions it catches itself, so everything is also
+# built with wasm exceptions (and the matching setjmp/longjmp).
+ENV CFLAGS="-I$INSTALL_DIR/include -pthread -fwasm-exceptions $EXTRA_CFLAGS"
 ENV CXXFLAGS="$CFLAGS"
 ENV LDFLAGS="-L$INSTALL_DIR/lib $CFLAGS $EXTRA_LDFLAGS"
 ENV EM_PKG_CONFIG_PATH=$INSTALL_DIR/lib/pkgconfig:/emsdk/upstream/emscripten/system/lib/pkgconfig

tests/unit/core/issues/fuzz-zscale-exceptions.test.js

@@ -0,0 +1,14 @@
+// Found by `npm run fuzz`: zimg (the zscale filter) reports errors by
+// throwing C++ exceptions it catches itself. The core was built without
+// exception support, so any zscale error aborted the whole module.
+import { useCore, execLogged } from "../../helpers.js";
+
+export default function () {
+  const ctx = useCore();
+
+  it("an invalid zscale size is an error, not a crash", () => {
+    const run = (vf) => execLogged(ctx.core, "-f", "lavfi", "-i", "color=c=red:s=64x48:d=0.3", "-vf", vf, "-f", "null", "-").ret;
+    expect(() => run("zscale=w=3:h=-1")).to.not.throw();
+    expect(run("zscale=w=32:h=24")).to.equal(0);
+  });
+}