config: add box-drawings-uses-font-glyphs=no|yes option

When disabled, we render box drawing characters ourselves. This is the
default.

When enabled, we instead use font glyphs. I.e. no special treatment.

Closes #430

config.h

@@ -99,6 +99,8 @@ struct config {
     struct pt_or_px horizontal_letter_offset;
     struct pt_or_px vertical_letter_offset;
 
+    bool box_drawings_uses_font_glyphs;
+
     struct {
         int lines;