[hb-view] Change subpixel bits from 8 to 6

To match FreeType units, such that FreeType gets correct size from us.
This matters more now that we allow setting --ft-load-flags.

util/hb-view.cc

@@ -30,7 +30,7 @@
 #include "view-cairo.hh"
 
 #define DEFAULT_FONT_SIZE 256
-#define SUBPIXEL_BITS 8
+#define SUBPIXEL_BITS 6
 
 int
 main (int argc, char **argv)