[Feature Request] add perspective pruning to cvHaarDetectObjects() (patch attached) (Patch #826)

For a given camera setup the y-coordinate of the horizon may be known. If the objects to be detected are attached to the ground, one can reduce the search space dramatically, because the search window must always be close to the horizon.

If the flag for perspective pruning is set, the y-range for the sliding window is limited to the proximity of the given horizon. The proximity is defined by up to three parameters: a fixed margin given in pixels, and two relative margins given in percentage of the window height. One margin for the maximum pixel distance above the horizon and one for the maximum distance below the horizon. (one could argue if one is enough, though)