options: transition options from OPT_FLAG to OPT_BOOL

c784820454 introduced a bool option type
as a replacement for the flag type, but didn't actually transition and
remove the flag type because it would have been too much mundane work.

sub/osd.h

@@ -153,8 +153,8 @@ struct osd_style_opts {
     int align_x;
     int align_y;
     float blur;
-    int bold;
-    int italic;
+    bool bold;
+    bool italic;
     int justify;
     int font_provider;
 };