Add a way to locate main image within associated images

[bgilbert]

Viewer applications would like to know where the main image (or images; see #99) is placed within the thumbnail or macro associated images (discussion). Several formats appear to record this information.

[Markus-PP]

Thanks for the feature hint. We will record this information in future, too.
As for the implementation - since any single of the associated images can possibly be located relative to the main image in any way, I guess an API expansion seems to be necessary for this, such as

void openslide_get_associated_image_coordinates (openslide_t *osr, const char *name, double *x, double *y, double *w, double *h);
Gets the location of an associated image relative to the main slide, level 0. If not applicable because coordinates don't exist, it returns -1 for all parameters.