pen: First check whether the in/out edges lie within the single pen vertex

In order to prevent underflow when searching for the closing pen vertex,
we first need to be sure that it does not simply lie next to the opening
pen vertex. As a result we were missing many cases that should have been
a bevel (in == out) and generating almost complete round caps instead.

Reported-by: Dominik Röttsches <dominik.rottsches@intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=56432
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

src/cairo-pen.c

@@ -412,21 +412,23 @@ _cairo_pen_find_active_cw_vertices (const cairo_pen_t *pen,
 	    i = 0;
     *start = i;
 
-    lo = i;
-    hi = i + pen->num_vertices;
-    i = (lo + hi) >> 1;
-    do {
-	int j = i;
-	if (j >= pen->num_vertices)
-	    j -= pen->num_vertices;
-	if (_cairo_slope_compare (&pen->vertices[j].slope_cw, out) > 0)
-	    hi = i;
-	else
-	    lo = i;
+    if (_cairo_slope_compare (out, &pen->vertices[i].slope_ccw) > 0) {
+	lo = i;
+	hi = i + pen->num_vertices;
 	i = (lo + hi) >> 1;
-    } while (hi - lo > 1);
-    if (i >= pen->num_vertices)
-	i -= pen->num_vertices;
+	do {
+	    int j = i;
+	    if (j >= pen->num_vertices)
+		j -= pen->num_vertices;
+	    if (_cairo_slope_compare (&pen->vertices[j].slope_cw, out) > 0)
+		hi = i;
+	    else
+		lo = i;
+	    i = (lo + hi) >> 1;
+	} while (hi - lo > 1);
+	if (i >= pen->num_vertices)
+	    i -= pen->num_vertices;
+    }
     *stop = i;
 }
 
@@ -452,20 +454,22 @@ _cairo_pen_find_active_ccw_vertices (const cairo_pen_t *pen,
 	    i = 0;
     *start = i;
 
-    lo = i;
-    hi = i + pen->num_vertices;
-    i = (lo + hi) >> 1;
-    do {
-	int j = i;
-	if (j >= pen->num_vertices)
-	    j -= pen->num_vertices;
-	if (_cairo_slope_compare (out, &pen->vertices[j].slope_ccw) > 0)
-	    hi = i;
-	else
-	    lo = i;
+    if (_cairo_slope_compare (&pen->vertices[i].slope_cw, out) < 0) {
+	lo = i;
+	hi = i + pen->num_vertices;
 	i = (lo + hi) >> 1;
-    } while (hi - lo > 1);
-    if (i >= pen->num_vertices)
-	i -= pen->num_vertices;
+	do {
+	    int j = i;
+	    if (j >= pen->num_vertices)
+		j -= pen->num_vertices;
+	    if (_cairo_slope_compare (out, &pen->vertices[j].slope_ccw) > 0)
+		hi = i;
+	    else
+		lo = i;
+	    i = (lo + hi) >> 1;
+	} while (hi - lo > 1);
+	if (i >= pen->num_vertices)
+	    i -= pen->num_vertices;
+    }
     *stop = i;
 }