Bug 1600470 - Reduce the emboldening strength used for synthetic-bold…

… faces with FreeType. r=lsalzman

Differential Revision: https://phabricator.services.mozilla.com/D57246

[wrupdater] From https://hg.mozilla.org/mozilla-central/rev/bf28a7e8a8a462ad596746232ab743dd892aa7b3

webrender/src/platform/unix/font.rs

@@ -106,7 +106,7 @@ ft_dyn_fn!(FT_Done_MM_Var(library: FT_Library, desc: *mut FT_MM_Var) -> FT_Error
 ft_dyn_fn!(FT_Set_Var_Design_Coordinates(face: FT_Face, num_vals: FT_UInt, vals: *mut FT_Fixed) -> FT_Error);
 
 extern "C" {
-    fn FT_GlyphSlot_Embolden(slot: FT_GlyphSlot);
+    fn mozilla_GlyphSlot_Embolden_Less(slot: FT_GlyphSlot);
 }
 
 enum FontFile {
@@ -495,7 +495,7 @@ impl FontContext {
         assert!(slot != ptr::null_mut());
 
         if font.flags.contains(FontInstanceFlags::SYNTHETIC_BOLD) {
-            unsafe { FT_GlyphSlot_Embolden(slot) };
+            unsafe { mozilla_GlyphSlot_Embolden_Less(slot) };
         }
 
         let format = unsafe { (*slot).format };