[base] Fix mono bitmap presetting (#53896).

It is rather fundamental to set monochrome bitmap based on rounded
CBox because the b/w rasterizer turns on pixels when their centers are
inside the glyph outline. The dropout control is unpredictable and can
distort narrow glyphs if the bitmap is too wide.

Reported by Chris Liddell.

* src/base/ftobjs.c (ft_glyphslot_preset_bitmap): If BBox boundaries
are too close, adjust them before rounding.

ChangeLog

@@ -1,3 +1,17 @@
+2018-05-15  Alexei Podtelezhnikov  <apodtele@gmail.com>
+
+	[base] Fix mono bitmap presetting (#53896).
+
+	It is rather fundamental to set monochrome bitmap based on rounded
+	CBox because the b/w rasterizer turns on pixels when their centers are
+	inside the glyph outline. The dropout control is unpredictable and can
+	distort narrow glyphs if the bitmap is too wide.
+
+	Reported by Chris Liddell.
+
+	* src/base/ftobjs.c (ft_glyphslot_preset_bitmap): If BBox boundaries
+	are too close, adjust them before rounding.
+
 2018-05-15  Werner Lemberg  <wl@gnu.org>
 
 	[psaux] Fix compiler warning (#53915).

src/base/ftobjs.c

@@ -374,32 +374,26 @@
       /* unless the rounded box can collapse for a narrow glyph */
       if ( cbox.xMax - cbox.xMin < 64 )
       {
-        cbox.xMin = FT_PIX_FLOOR( cbox.xMin );
-        cbox.xMax = FT_PIX_CEIL_LONG( cbox.xMax );
-      }
-      else
-      {
-        cbox.xMin = FT_PIX_ROUND_LONG( cbox.xMin );
-        cbox.xMax = FT_PIX_ROUND_LONG( cbox.xMax );
+        cbox.xMin = ( cbox.xMin + cbox.xMax ) / 2 - 32;
+        cbox.xMax = cbox.xMin + 64;
       }
 
+      cbox.xMin = FT_PIX_ROUND_LONG( cbox.xMin );
+      cbox.xMax = FT_PIX_ROUND_LONG( cbox.xMax );
+
       if ( cbox.yMax - cbox.yMin < 64 )
       {
-        cbox.yMin = FT_PIX_FLOOR( cbox.yMin );
-        cbox.yMax = FT_PIX_CEIL_LONG( cbox.yMax );
-      }
-      else
-      {
-        cbox.yMin = FT_PIX_ROUND_LONG( cbox.yMin );
-        cbox.yMax = FT_PIX_ROUND_LONG( cbox.yMax );
+        cbox.yMin = ( cbox.yMin + cbox.yMax ) / 2 - 32;
+        cbox.yMax = cbox.yMin + 64;
       }
+
+      cbox.yMin = FT_PIX_ROUND_LONG( cbox.yMin );
+      cbox.yMax = FT_PIX_ROUND_LONG( cbox.yMax );
+
+      break;
 #else
-      cbox.xMin = FT_PIX_FLOOR( cbox.xMin );
-      cbox.yMin = FT_PIX_FLOOR( cbox.yMin );
-      cbox.xMax = FT_PIX_CEIL_LONG( cbox.xMax );
-      cbox.yMax = FT_PIX_CEIL_LONG( cbox.yMax );
+      goto Round;
 #endif
-      break;
 
     case FT_RENDER_MODE_LCD:
       pixel_mode = FT_PIXEL_MODE_LCD;