Refactor mutator class

[cyanglaz]

All the mutators are currently represented in a single Mutator class : https://github.com/flutter/engine/blob/main/flow/embedded_views.h#L38
The Mutator class has a union member that contains local variables for different types of mutators.

This causes some issue:

1. Pointers in unions need to manually managed and it is error prone.
2. Storing a copy of a scalar object creates overhead.

We can refactor the Mutator class in the following way so that each type of the mutator stores shared_pointers of the wrapped object separately.

1. Make the Mutator class abstract.
2. Mutator class contains a type method that gets the type of the mutator.
3. Make each type of Mutator a subclass of Mutator, for example, class TransformMutator : public Mutator
4. Mutator class contains a asXXX method that casting the object to the particular mutator. For example, asClipRect returns a ClipRectMutator object.
5. Each subclass of Mutator contains a shared_ptr member that represents the content of the mutator.

[github-actions]

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