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/effects/video_effect.cpp

@@ -0,0 +1,47 @@
+#include "effects/video_effect.hpp"
+
+#include <algorithm>
+#include <cmath>
+
+#include "core/video.hpp"
+#include "effects/source_fill.hpp"
+
+namespace effector {
+
+double Moment::length() const noexcept {
+    return static_cast<double>(std::max(1, frame_.count)) / std::max(1, frame_.fps);
+}
+
+double Moment::loopRate(double perSecond) const noexcept {
+    const double clip = length();
+    return std::max(1.0, std::round(perSecond * clip)) / clip;
+}
+
+double Moment::cycle(double perSecond) const noexcept {
+    return std::fmod(seconds() * loopRate(perSecond), 1.0);
+}
+
+Rng Moment::fresh() const noexcept {
+    return Rng(seed_ ^ (0x9e3779b97f4a7c15ULL * static_cast<std::uint64_t>(frame_.index + 1)));
+}
+
+Image VideoEffect::render(const std::vector<Image>& sources, int width, int height, Rng& rng,
+                          const ParamValues& values) const {
+    return renderFrame(sources, width, height, rng, values, FrameTime{0, kDefaultFps, kDefaultFps});
+}
+
+Image VideoEffect::renderFrame(const std::vector<Image>& sources, int width, int height, Rng& rng,
+                               const ParamValues& values, const FrameTime& frame) const {
+    if (width < 1 || height < 1) throw EffectError("Canvas size must be at least 1x1 pixels.");
+    const FrameTime at = frame.video() ? frame : FrameTime{0, kDefaultFps, kDefaultFps};
+
+    // Both of these take the same values from the shared rng on every frame:
+    // the crop a leading stage starts from, and the one seed everything the
+    // effect draws comes from.
+    Image input = canvasFromSources(sources, width, height, rng);
+    const std::uint64_t seed = rng.next();
+
+    return animate(std::move(input), values, Moment(at, seed));
+}
+
+}  // namespace effector