Add Film gate, a video effect of film running through a projector

Film grain's field moves to effects/grain_field.hpp so both share it; Film
grain renders exactly as before.

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

src/effects/filmgate.hpp

@@ -0,0 +1,28 @@
+#pragma once
+
+#include "effects/video_effect.hpp"
+
+namespace effector {
+
+// Film running through a projector: the grain is new on every frame, the
+// picture sways a little in the gate and its brightness flickers, and dust
+// and scratches pass over it.
+//
+// The sway is a sum of slow swings, each a whole number of times in the
+// clip, so it loops; grain, flicker and dust are drawn from at.fresh() and
+// are different on every frame, as they are on film.
+class FilmGateEffect : public VideoEffect {
+public:
+    std::string name() const override { return "filmgate"; }
+    std::string title() const override { return "Film gate"; }
+    std::string description() const override {
+        return "Film running through a projector: grain new on every frame, as Film grain draws "
+               "it, a picture swaying in the gate, flicker, dust and scratches.";
+    }
+    const std::vector<ParamSpec>& params() const override;
+
+protected:
+    Image animate(Image input, const ParamValues& values, const Moment& at) const override;
+};
+
+}  // namespace effector