Support rotation/scale/flip for Actor

Originally reported by: **Daniel Pope (Bitbucket: [lordmauve](https://bitbucket.org/lordmauve), GitHub: [lordmauve](https://github.com/lordmauve))**

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Pygame supports somewhat slow rotation, scale and flip operations in software.

We should expose these for the Actor class.

May depend on issue #2, which allows users to specify an "anchor point" around which the actor will rotate.

Open questions: how should these operations affect the actor's bounding rect?

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- Bitbucket: https://bitbucket.org/lordmauve/pgzero/issue/3

Start with the Actor class and review how Pygame provides rotation, scaling, and flipping in software. Read issue #2 for the proposed anchor-point behavior, then resolve how these operations should affect the actor's bounding rect. Done means the Actor API exposes all three operations with a defined bounding-rect behavior.

Written by the indexing model from the issue text.