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returns one of the four
EDGE_NOT_SEEN if the edge is outside the rendering window
in this case p1,p2 == clipped_p1,clipped_p2
EDGE_FULLY_SEEN if the edge is fully inside the rendering window
in this case p1,p2 == clipped_p1,clipped_p2
EDGE_PARTIALLY_SEEN if some of the edge is inside the window
in this case clipped_p1,clipped_p2 is the seen part
VR_ERROR an error occurred
in this case clipped_p1,clipped_p2 are not valid
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