Allow 68K to return mid-instruction. Adjust how 68K interrupt ack works so int2 busy flag timing is more correct. Fix some other SCD timing issues

#ifndef PPM_H_
#define PPM_H_

void save_ppm(FILE *f, uint32_t *buffer, uint32_t width, uint32_t height, uint32_t pitch);

#endif //PPM_H_