[smooth] Fix GCC LTO crashes on Windows.

Fixes #1164 by using a volatile variable around `setjmp`.  It is hard to
say how this fixes crashes related to certain link-time optimizations.
This does not decrease the rendering performance.

* src/smooth/ftgrays.c (gray_convert_glyph_inner): Use volatile `error`.
diff --git a/src/smooth/ftgrays.c b/src/smooth/ftgrays.c
index bff3c59..28a081c 100644
--- a/src/smooth/ftgrays.c
+++ b/src/smooth/ftgrays.c
@@ -1912,7 +1912,7 @@
   gray_convert_glyph_inner( RAS_ARG_
                             int  continued )
   {
-    int  error;
+    volatile int  error;
 
 
     if ( ft_setjmp( ras.jump_buffer ) == 0 )