Split effects into image and video effects, with VHS tracking as the first video effect

Effect::video() says which kind an effect is. The Add effect menu lists the
two kinds in sections of their own, each scrolling on its own, and offers
video effects only in a video project; a video stage carries a film icon in
the chain and, in a still, is dimmed and skipped like a stage switched off.
enabledStages takes whether the render is a video and is the one place that
leaves video stages out of a still. --help groups the video effects under a
heading of their own, --list-effects marks them, and a video effect given
for a still is skipped with a warning.

VideoEffect is the base every video effect builds on, and keeps the rules a
moving stage has to follow: it takes one value from the shared rng, the same
on every frame, and hands the effect steady and per-frame generators and
loop-rounded time through a Moment, so a subclass writes only animate().
VHS tracking is the first, and the one to copy: the colour planes sway and
shake and a tracking bar rolls down the picture.

That motion moves out of channel split, which is an image effect again and
renders stills exactly as before; forVideo() goes with it. Version 0.7.0.

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

src/core/effect.hpp

@@ -58,21 +58,22 @@ public:
         return title();
     }
 
-    // Whether a stage with these values changes from one frame of a video to
-    // the next. A stage that does not is rendered once for the whole clip.
-    virtual bool animated(const ParamValues& values) const {
-        (void)values;
-        return false;
-    }
+    // Whether this is a video effect rather than an image effect. An image
+    // effect makes a picture and holds still in a clip; a video effect is
+    // motion and nothing else, so it is offered only in a video project and a
+    // still skips it whole, as it skips a stage that is switched off. Video
+    // effects derive from VideoEffect (effects/video_effect.hpp), which
+    // answers true here and keeps the rules a moving stage has to follow.
+    virtual bool video() const { return false; }
 
     virtual Image render(const std::vector<Image>& sources, int width, int height, Rng& rng,
                          const ParamValues& values) const = 0;
 
-    // One frame of a video, or a still when `frame` says so. An effect that
-    // does not move needs only render(), which this falls back to. One that
-    // does must leave `rng` in the same state on every frame -- a frame's own
-    // randomness comes from a generator of its own -- so the stages after it
-    // stay put while it moves.
+    // One frame of a video, or a still when `frame` says so. An image effect
+    // needs only render(), which this falls back to; a video effect renders
+    // here, and must leave `rng` in the same state on every frame -- a
+    // frame's own randomness comes from a generator of its own -- so the
+    // stages after it stay put while it moves.
     virtual Image renderFrame(const std::vector<Image>& sources, int width, int height, Rng& rng,
                               const ParamValues& values, const FrameTime& frame) const {
         (void)frame;