Bring more of the Velato runtime, including Composition, Renderer. (#15)

This code will need a lot of enhancements and improvements, but it is
still very close to what was in Velato with only minimal changes.

This brings an optional dependency on Vello so that the `Renderer` can
be used to create a Vello `Scene` from the animation and composition
data.

Author: waywardmonkeys

.github/workflows/ci.yml

@@ -13,7 +13,7 @@ env:
   # This should be limited to packages that are intended for publishing.
   RUST_MIN_VER_PKGS: "-p interpoli"
   # List of features that depend on the standard library and will be excluded from no_std checks.
-  FEATURES_DEPENDING_ON_STD: "std,default"
+  FEATURES_DEPENDING_ON_STD: "std,default,vello"
 
 
 # Rationale