Simplify image transforms

Simplify image transforms

* src/image.c: (image_set_rotation, image_set_size,
image_set_transform): Combine into image_set_transform.
(image_set_crop): Remove function.
(lookup_image): Remove calls to removed functions and remove
transform_matrix.
* test/manual/image-transforms-tests.el (test-cropping): Remove
function.
(test-transforms): Remove reference to test-cropping.