Show U+FFFD REPLACEMENT CHARACTER for invalid Unicode codepoints

Only if the font doesn't support it.  Ie, this gives the user to
use non-Unicode codepoints as private values and return a meaningful
glyph for them.  But if it's invalid and font callback doesn't
like it, and if font has U+FFFD, show that instead.

Font functions that do not want this automatic replacement to
happen should return true from get_glyph() if unicode > 0x10FFFF.

Replaces #27

src/hb-ot-shape-normalize.cc

@@ -209,7 +209,14 @@ decompose_current_character (const hb_ot_shape_normalize_context_t *c, bool shor
   else if (decompose_compatibility (c, buffer->cur().codepoint))
     skip_char (buffer);
   else
+  {
+    /* Not found, not decomposible;  If codepoint is invalid Unicode and
+     * font supports U+FFFD REPLACEMENT CHARACTER, use that instead. */
+    hb_codepoint_t FFFD_glyph;
+    if (buffer->cur().codepoint > 0x10FFFF && c->font->get_glyph (0xFFFD, 0, &FFFD_glyph))
+      glyph = FFFD_glyph;
     next_char (buffer, glyph); /* glyph is initialized in earlier branches. */
+  }
 }
 
 static inline void