Clear font_positions if there is no font.txt

This fixes a bug where font_positions wouldn't get cleared after exiting
a custom level that had a font.txt if it didn't exist in the default
graphics, leading to messed-up-looking font rendering.

This bug was originally discovered by Ally.

desktop_version/src/Graphics.cpp

@@ -267,6 +267,8 @@ void Graphics::Makebfont()
             ++pos;
         }
         FILESYSTEM_freeMemory(&charmap);
+    } else {
+        font_positions.clear();
     }
 }