Projective transformations interface

The abstract type ProjectiveTransform represents a projective transformation. A ProjectiveTransform needs to implement getprojection(tfm, bounds; randstate) that should return a Transformation from CoordinateTransformations.jl.

To add support for projective transformations to an item I, you need to implement

• getbounds(item::I) returns the spatial bounds of the item; and
• project(P, item::I, indices) applies the projective transformation P and crops to indices

To support apply!-ing projective transformations, project!(bufitem, P, item) can also be implemented.