Fix overflow issues in image rotation

Fix overflow issues in image rotation

Also, do some refactoring to simplify code.
* src/dispextern.h (INIT_MATRIX, COPY_MATRIX, MULT_MATRICES): Remove.
* src/image.c (matrix3x3): New type, replacing all uses of 3x3 double.
(matrix3x3_copy, matrix3x3_mult): New functions, replacing
COPY_MATRIX, MULT_MATRICES.  Replace INIT_MATRIX by C initializers.
(image_set_rotation): Use Fmod to avoid undefined behavior on
double-to-int conversion and to reduce bignum rotations correctly.
(image_set_crop): Finish up previous correction, by not re-setting
width and height if compute_image_size has set them.
Prefer shifting right by 1 to dividing by 2 if either will do.