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>

CHANGELOG.md

@@ -4,6 +4,15 @@ Every release of Effector, newest first. The version lives in one place, the
 `project()` line of `CMakeLists.txt`, and each release is a `vX.Y.Z` tag on the
 commit that made it. The interface shows this file under Changelog in its menu.
 
+## [0.8.0] - 2026-09-26
+
+### Added
+- Tile swap, a video effect: tiles cut from the photographs are laid over the
+  canvas as Tiles lays them, and each one is swapped for another cut at a
+  moment of its own, so the mosaic keeps changing piece by piece. On the
+  command line it is `-e tileswap` with `--tileswap-rate`,
+  `--tileswap-layout` and the rest.
+
 ## [0.7.0] - 2026-09-26
 
 ### Added