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.

Notes:
Thu Jul 7 08:02:17 EDT 2005 rsc
    please send patches that are just the change you want to make.
    deleting that comment meant i had to work extra hard to preserve it.
    
    thanks.


Reference: /n/sources/patch/applied/linesrcalign
Date: Mon Jul  4 20:40:46 CES 2005
Reviewed-by: rsc

--- /sys/src/libmemdraw/line.c	Mon Jul  4 20:38:19 2005
+++ /sys/src/libmemdraw/line.c	Mon Jul  4 20:38:15 2005
@@ -315,12 +315,6 @@
 void
 _memimageline(Memimage *dst, Point p0, Point p1, int end0, int end1, int radius, Memimage *src, Point sp, Rectangle clipr, int op)
 {
-	/*
-	 * BUG: We should really really pick off purely horizontal and purely
-	 * vertical lines and handle them separately with calls to memimagedraw
-	 * on rectangles.
-	 */
-
 	int hor;
 	int sin, cos, dx, dy, t;
 	Rectangle oclipr, r;
@@ -369,6 +363,7 @@
 		}
 		oclipr = dst->clipr;
 		dst->clipr = clipr;
+		sp = addpt(r.min, d);
 		memimagedraw(dst, r, src, sp, memopaque, sp, op);
 		dst->clipr = oclipr;
 		return;