fixed the p0/sp correspondence for the optimized case
(horizontal/vertical).

	line(dst, p0, p1, end0, end1, thick, src, sp)
		Line draws in dst a line of width 1+2*thick pixels
		joining points p0 and p1. The line is drawn using pix-
		els from the src image aligned so sp in the source cor-
		responds to p0 in the destination.