[raster] Handle drop-outs at glyph borders according to Microsoft.

If a drop-out rule would switch on a pixel outside of the glyph's
bounding box, use the right (or top) pixel instead.  This is an
undocumented feature, but some fonts like `Helvetica Neue LT Com 65
Medium' heavily rely on it.

Thanks to Greg Hitchcock who explained this behaviour.

* src/raster/ftraster.c (Vertical_Sweep_Drop,
Horizontal_Sweep_Drop): Implement it.

ChangeLog

@@ -1,3 +1,17 @@
+2011-01-13  Werner Lemberg  <wl@gnu.org>
+
+	[raster] Handle drop-outs at glyph borders according to Microsoft.
+
+	If a drop-out rule would switch on a pixel outside of the glyph's
+	bounding box, use the right (or top) pixel instead.  This is an
+	undocumented feature, but some fonts like `Helvetica Neue LT Com 65
+	Medium' heavily rely on it.
+
+	Thanks to Greg Hitchcock who explained this behaviour.
+
+	* src/raster/ftraster.c (Vertical_Sweep_Drop,
+	Horizontal_Sweep_Drop): Implement it.
+
 2011-01-09  suzuki toshiya  <mpsuzuki@hiroshima-u.ac.jp>
 
 	[cache] Fix Savannah bug #31923, patch drafted by Harsha.

src/raster/ftraster.c

@@ -2403,6 +2403,11 @@
           return;  /* no drop-out control */
         }
 
+        /* undocumented but confirmed: If the drop-out would result in a  */
+        /* pixel outside of the bounding box, use the right pixel instead */
+        if ( pxl < 0 )
+          pxl = e1;
+
         /* check that the other pixel isn't set */
         e1 = pxl == e1 ? e2 : e1;
 
@@ -2579,6 +2584,11 @@
           return;  /* no drop-out control */
         }
 
+        /* undocumented but confirmed: If the drop-out would result in a */
+        /* pixel outside of the bounding box, use the top pixel instead  */
+        if ( pxl < 0 )
+          pxl = e1;
+
         /* check that the other pixel isn't set */
         e1 = pxl == e1 ? e2 : e1;