tessellator bug fix: in-fill-empty-trapezoid

The cairo_in_fill() function sometimes gives false positives
when it samples a point on the edge of an empty trapezoid.
This patch alleviates the bug (but doesn't fix it completely),
for the common(?) case where the left and right edges of the
empty trapezoid have equal top and bottom points.

src/cairo-bentley-ottmann.c

@@ -1078,6 +1078,7 @@ _cairo_bo_edge_end_trap (cairo_bo_edge_t	*left,
     fixed_top = trap->top >> CAIRO_BO_GUARD_BITS;
     fixed_bot = bot >> CAIRO_BO_GUARD_BITS;
 
+    /* Only emit trapezoids with positive height. */
     if (fixed_top < fixed_bot) {
 	cairo_point_t left_top, left_bot, right_top, right_bot;
 
@@ -1090,19 +1091,29 @@ _cairo_bo_edge_end_trap (cairo_bo_edge_t	*left,
 	right_bot.x = right->bottom.x >> CAIRO_BO_GUARD_BITS;
 	right_bot.y = right->bottom.y >> CAIRO_BO_GUARD_BITS;
 
-	status = _cairo_traps_add_trap_from_points (bo_traps->traps,
-						    fixed_top,
-						    fixed_bot,
-						    left_top, left_bot,
-						    right_top, right_bot);
+	/* Avoid emitting the trapezoid if it is obviously degenerate.
+	 * TODO: need a real collinearity test here for the cases
+	 * where the trapezoid is degenerate, yet the top and bottom
+	 * coordinates aren't equal.  */
+	if (left_top.x != right_top.x ||
+	    left_top.y != right_top.y ||
+	    left_bot.x != right_bot.x ||
+	    left_bot.y != right_bot.y)
+	{
+	    status = _cairo_traps_add_trap_from_points (bo_traps->traps,
+							fixed_top,
+							fixed_bot,
+							left_top, left_bot,
+							right_top, right_bot);
 
 #if DEBUG_PRINT_STATE
-	printf ("Deferred trap: left=(%08x, %08x)-(%08x,%08x) "
-		"right=(%08x,%08x)-(%08x,%08x) top=%08x, bot=%08x\n",
-		left->top.x, left->top.y, left->bottom.x, left->bottom.y,
-		right->top.x, right->top.y, right->bottom.x, right->bottom.y,
-		trap->top, bot);
+	    printf ("Deferred trap: left=(%08x, %08x)-(%08x,%08x) "
+		    "right=(%08x,%08x)-(%08x,%08x) top=%08x, bot=%08x\n",
+		    left->top.x, left->top.y, left->bottom.x, left->bottom.y,
+		    right->top.x, right->top.y, right->bottom.x, right->bottom.y,
+		    trap->top, bot);
 #endif
+	}
     }
 
     _cairo_freelist_free (&bo_traps->freelist, trap);