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/GfxRenderer/GfxRenderer.cpp

@@ -582,7 +582,7 @@ void GfxRenderer::drawTextRotated90CW(const int fontId, const int x, const int y
   while ((cp = utf8NextCodepoint(reinterpret_cast<const uint8_t**>(&text)))) {
     const EpdGlyph* glyph = font.getGlyph(cp, style);
     if (!glyph) {
-      glyph = font.getGlyph('?', style);
+      glyph = font.getGlyph(REPLACEMENT_GLYPH, style);
     }
     if (!glyph) {
       continue;
@@ -760,8 +760,7 @@ void GfxRenderer::renderChar(const EpdFontFamily& fontFamily, const uint32_t cp,
                              const bool pixelState, const EpdFontFamily::Style style) const {
   const EpdGlyph* glyph = fontFamily.getGlyph(cp, style);
   if (!glyph) {
-    // TODO: Replace with fallback glyph property?
-    glyph = fontFamily.getGlyph('?', style);
+    glyph = fontFamily.getGlyph(REPLACEMENT_GLYPH, style);
   }
 
   // no glyph?