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>

CMakeLists.txt

@@ -1,5 +1,5 @@
 cmake_minimum_required(VERSION 3.20)
-project(effector VERSION 0.6.1 LANGUAGES CXX)
+project(effector VERSION 0.7.0 LANGUAGES CXX)
 
 set(CMAKE_CXX_STANDARD 20)
 set(CMAKE_CXX_STANDARD_REQUIRED ON)
@@ -56,6 +56,8 @@ add_library(effector_core STATIC
     src/effects/slitshift.cpp
     src/effects/text.cpp
     src/effects/tiles.cpp
+    src/effects/tracking.cpp
+    src/effects/video_effect.cpp
     src/effects/warp.cpp
 )
 target_include_directories(effector_core PUBLIC src ${CMAKE_CURRENT_BINARY_DIR}/generated)