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libmemdraw: handle memarc() phi == 0 and phi <= -360, keep alpha in bounds

--- a/sys/src/libmemdraw/arc.c

+++ b/sys/src/libmemdraw/arc.c

@@ -40,13 +40,13 @@

 	Point p,	bnd[8];

 	Memimage *wedge, *figure, *mask;

-	if(a < 0)

-		a = -a;

-	if(b < 0)

-		b = -b;

-	w = t;

-	if(w < 0)

-		w = 0;

+	if(phi == 0)

+		return;

+	if(phi <= -360 || phi >= 360){

+		memellipse(dst, c, a, b, t, src, sp, op);

+		return;

+	}

+	alpha %= 360;

 	alpha = -alpha;		/* compensate for upside-down coords */

 	phi = -phi;

 	beta = alpha + phi;

@@ -56,10 +56,6 @@

 		beta = tmp;

 		phi = -phi;

 	}

-	if(phi >= 360){

-		memellipse(dst, c, a, b, t, src, sp, op);

-		return;

-	}

 	while(alpha < 0)

 		alpha += 360;

 	while(beta < 0)

@@ -70,6 +66,15 @@

 		 * icossin returns point at radius ICOSSCALE.

 		 * multiplying by m1 moves it outside the ellipse

 		*/

+

+	if(a < 0)

+		a = -a;

+	if(b < 0)

+		b = -b;

+	w = t;

+	if(w < 0)

+		w = 0;

+

 	rect = Rect(-a-w, -b-w, a+w+1, b+w+1);

 	m = rect.max.x;	/* inradius of bounding square */

 	if(m < rect.max.y)