box-drawing: add infrastructure for rendering box drawing characters ourselves

* ‘term’ struct contains an array of 160 fcft glyph pointers
* the glyph pointers are lazily allocated when we need to draw a box
  drawings character
* Filtering out box drawings characters is easy - they are (except
  unicode 13, which isn’t handled yet )all in a single range.

box-drawing.h

@@ -0,0 +1,6 @@
+#pragma once
+
+#include <fcft/fcft.h>
+
+struct terminal;
+struct fcft_glyph *box_drawing(struct terminal *term, wchar_t wc);