Auto merge of #3432 - lsalzman:pad-bb, r=kvark

ensure lcd filter padding is applied to cbox when rasterizing glyphs with FreeType

This resolves Gecko bug https://bugzilla.mozilla.org/show_bug.cgi?id=1508163

We don't apply the lcd filter padding when querying the cbox to rasterize the glyph, even though we apply the padding when querying the bounds. This causes subpixel AA text to get shifted by the amount of the padding due to the padding not being accounted for in the offset.

The solution is to apply the padding in both cases so it properly cancels out.

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Author: bors-servo

webrender/src/platform/unix/font.rs

@@ -369,6 +369,20 @@ impl FontContext {
         }
     }
 
+    fn pad_bounding_box(&self, font: &FontInstance, cbox: &mut FT_BBox) {
+        // Apply extra pixel of padding for subpixel AA, due to the filter.
+        if font.render_mode == FontRenderMode::Subpixel {
+            let padding = (self.lcd_extra_pixels * 64) as FT_Pos;
+            if font.flags.contains(FontInstanceFlags::LCD_VERTICAL) {
+                cbox.yMin -= padding;
+                cbox.yMax += padding;
+            } else {
+                cbox.xMin -= padding;
+                cbox.xMax += padding;
+            }
+        }
+    }
+
     // Get the bounding box for a glyph, accounting for sub-pixel positioning.
     fn get_bounding_box(
         &self,
@@ -389,17 +403,7 @@ impl FontContext {
             }
         }
 
-        // Apply extra pixel of padding for subpixel AA, due to the filter.
-        if font.render_mode == FontRenderMode::Subpixel {
-            let padding = (self.lcd_extra_pixels * 64) as FT_Pos;
-            if font.flags.contains(FontInstanceFlags::LCD_VERTICAL) {
-                cbox.yMin -= padding;
-                cbox.yMax += padding;
-            } else {
-                cbox.xMin -= padding;
-                cbox.xMax += padding;
-            }
-        }
+        self.pad_bounding_box(font, &mut cbox);
 
         // Offset the bounding box by subpixel positioning.
         // Convert to 26.6 fixed point format for FT.
@@ -572,6 +576,7 @@ impl FontContext {
             let outline = &(*slot).outline;
             let mut cbox: FT_BBox = mem::uninitialized();
             FT_Outline_Get_CBox(outline, &mut cbox);
+            self.pad_bounding_box(font, &mut cbox);
             FT_Outline_Translate(
                 outline,
                 dx - ((cbox.xMin + dx) & !63),