Add Tile swap, a video effect built from the source images

Tile swap lays tiles cut from the photographs over the canvas as Tiles
does, in a grid, in bricks or scattered, and swaps each one for another
cut at a moment of its own, so the mosaic keeps changing piece by piece.
The layout stays put for the whole clip; what a tile shows depends only
on the tile and how many times it has swapped, counted round the clip, so
it holds still between swaps and the clip loops without a jump. On the
command line it is -e tileswap with --tileswap-rate and the rest.

VideoEffect learns a second kind: an effect that answers needsSources()
writes compose() instead of animate(), and the base then takes no crop,
only the one seed, so it sees the photographs when it leads the chain and
the stage before it otherwise. Transforms render exactly as before. The
tile cutting and painting move out of tiles.cpp into tile_plan.hpp for
both effects to share; Tiles renders byte for byte as before. Version
0.8.0.

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 0.7.0 LANGUAGES CXX)
+project(effector VERSION 0.8.0 LANGUAGES CXX)
 
 set(CMAKE_CXX_STANDARD 20)
 set(CMAKE_CXX_STANDARD_REQUIRED ON)
@@ -56,6 +56,7 @@ add_library(effector_core STATIC
     src/effects/slitshift.cpp
     src/effects/text.cpp
     src/effects/tiles.cpp
+    src/effects/tileswap.cpp
     src/effects/tracking.cpp
     src/effects/video_effect.cpp
     src/effects/warp.cpp