Scale Composition: comp.scale resizes a comp and scales its layers, keys, cameras and guides by one factor

Like After Effects' File > Scripts > Scale Composition.jsx: a factor, or a new
width or height that keeps the aspect ratio, in one undo step. Layers whose
parent has a Scale follow that parent; nested precomps and expressions are
left as they are. No dialog yet.

Refs #2208

crates/engine/src/tests.rs

@@ -38,6 +38,7 @@ mod quick_offset;
 mod replace_source;
 mod rig3d;
 pub(crate) mod roto;
+mod scale_comp;
 mod sequence;
 mod session;
 mod settings;