Fixed size vector subtraction

To subtract 3-vectors use the equivalent macros and functions

      gan_vec3_sub_q ( &v3x, &v3y, &v3z ); /* macro */
      v3z = gan_vec3_sub_s ( &v3x, &v3y ); /* function call */
      gan_vec3_sub_i1 ( &v3x, &v3y ); /* result in-place in v3x */
      gan_vec3_sub_i2 ( &v3x, &v3y ); /* result in-place in v3y */
      gan_vec3_decrement ( &v3x, &v3y ); /* result in-place in v3x */