The
XtAddExposureToRegion function computes the union of the rectangle defined
by the exposure event and the specified region. Then, it stores the results
back in region. If the event argument is not an Expose or GraphicsExpose
event, XtAddExposureToRegion returns without an error and without modifying
region.
This function is used by the exposure compression mechanism (see
Section 7.9.3).