Adjust the width of various spaces if font does not cover them

See discussion here:
https://github.com/behdad/harfbuzz/commit/81ef4f407d9c7bd98cf62cef951dc538b13442eb

There's no way to disable this fallback, but I don't think it would
be needed.  Let's hope for the best!

Fixes https://github.com/behdad/harfbuzz/issues/153

src/hb-buffer.cc

diff --git a/src/hb-buffer.cc b/src/hb-buffer.cc
index 50710dd..f690769 100644
--- a/src/hb-buffer.cc
+++ b/src/hb-buffer.cc
@@ -198,6 +198,7 @@
 
   hb_segment_properties_t default_props = HB_SEGMENT_PROPERTIES_DEFAULT;
   props = default_props;
+  scratch_flags = HB_BUFFER_SCRATCH_FLAG_DEFAULT;
 
   content_type = HB_BUFFER_CONTENT_TYPE_INVALID;
   in_error = false;
@@ -738,6 +739,7 @@
     HB_BUFFER_FLAG_DEFAULT,
     HB_BUFFER_CLUSTER_LEVEL_DEFAULT,
     HB_BUFFER_REPLACEMENT_CODEPOINT_DEFAULT,
+    HB_BUFFER_SCRATCH_FLAG_DEFAULT,
 
     HB_BUFFER_CONTENT_TYPE_INVALID,
     HB_SEGMENT_PROPERTIES_DEFAULT,