fix: render U+FFFD replacement character instead of ? (#366)

The current behavior of rendering `?` for an unknown Unicode character
can be hard to distinguish from a typo. Use the standard Unicode
"replacement character" instead, that's what it's designed for:

https://en.wikipedia.org/wiki/Specials_(Unicode_block)

I'm making this PR as a draft because I'm not sure I did everything that
was needed to change the character set covered by the fonts. Running
that script is in its own commit. If this is proper, I'll rebase/squash
into one commit and un-draft.

Co-authored-by: Maeve Andrews <maeve@git.mail.maeveandrews.com>

lib/EpdFont/EpdFont.cpp

@@ -22,8 +22,7 @@ void EpdFont::getTextBounds(const char* string, const int startX, const int star
     const EpdGlyph* glyph = getGlyph(cp);
 
     if (!glyph) {
-      // TODO: Replace with fallback glyph property?
-      glyph = getGlyph('?');
+      glyph = getGlyph(REPLACEMENT_GLYPH);
     }
 
     if (!glyph) {