Updates dependencies to their latest versions.
Removes the feature directive for `if-let`.

The feature-gate for this has been removed, so the directive is no
longer necessary.
Adds `deriving(Copy)` for various structs.

`Copy` is no longer implemented by default, so the proper `deriving`
statement has been added for the `ColorRGB` and `Material` structs and
all the structs in the `geometry` module.