Fonts: Prefer using U+FFFD character for fallback instead of '?'. Use U+FF0E dot character to construct an ellipsis if U+002E '.' is not available. (#4269)

imgui.h

@@ -2709,7 +2709,8 @@ struct ImFont
     const ImFontConfig*         ConfigData;         // 4-8   // in  //            // Pointer within ContainerAtlas->ConfigData
     short                       ConfigDataCount;    // 2     // in  // ~ 1        // Number of ImFontConfig involved in creating this font. Bigger than 1 when merging multiple font sources into one ImFont.
     ImWchar                     FallbackChar;       // 2     // in  // = '?'      // Replacement character if a glyph isn't found. Only set via SetFallbackChar()
-    ImWchar                     EllipsisChar;       // 2     // out // = -1       // Character used for ellipsis rendering.
+    ImWchar                     EllipsisChar;       // 2     // out // = '...'    // Character used for ellipsis rendering.
+    ImWchar                     DotChar;            // 2     // out // = '.'      // Dot character used for fallback ellipsis rendering.
     bool                        DirtyLookupTables;  // 1     // out //
     float                       Scale;              // 4     // in  // = 1.f      // Base font scale, multiplied by the per-window font scale which you can adjust with SetWindowFontScale()
     float                       Ascent, Descent;    // 4+4   // out //            // Ascent: distance from top to bottom of e.g. 'A' [0..FontSize]