Added RenderCheckMark() helper following the font size

Pretty ugly, not sure what's the right way to solve that automatically,
better shown as an "icon" so it doesn't have to necessarily look like
the font.

imgui.h

@@ -1018,6 +1018,7 @@ struct ImFont
         signed short    XOffset, YOffset;
         float           U0, V0, U1, V1;     // Texture coordinates
     };
+    float               BaseLine;           // Distance from top to bottom of e.g. 'A' [0..FontSize]
     ImFontAtlas*        ContainerAtlas;     // What we has been loaded into
     ImVector<Glyph>     Glyphs;
     const Glyph*        FallbackGlyph;      // == FindGlyph(FontFallbackChar)