Changelog

Changed signatures of RotateCamera(), PanCamera(), and ZoomCamera() to make them straightforward to use from other code -- they now accept parameters specifying their operations in world units (or a multiplicative factor for zoom), rather than specifying the input events that precipitated the camera operations.
Added methods to compute parameters for rotations and pans from mouse events, and constants to specify zoom parameters.
Various minor code improvements
Fixed minor error in README

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