warm (or map) a unit square onto an arbitrary quadrilateral
according to the 4-point correspondences:
(0,0) -> (x0,y0)
(1,0) -> (x1,y1)
(1,1) -> (x2,y2)
(0,1) -> (x3,y3)
The mapping is done using perspective transformation which preserve
lines in all orientations and permit quadrilateral to quadrilateral
mappings. see ref. bellow.