message: Completely clear font when rebuilding atlas.
  Previously, IndexLookup was not cleared on each font, causing FindGlyph to return old glyphs when using MergeMode.
diff: @@ -1577,7 +1577,6 @@ bool ImFontAtlasBuildWithStbTruetype(ImFontAtlas* atlas)
  const float off_x = cfg.GlyphOffset.x;
  const float off_y = cfg.GlyphOffset.y + (float)(int)(dst_font->Ascent + 0.5f);
  - dst_font->FallbackGlyph = NULL; // Always clear fallback so FindGlyph can return NULL. It will be set again in BuildLookupTable()
  for (int i = 0; i < tmp.RangesCount; i++)
  {
  stbtt_pack_range& range = tmp.Ranges[i];
  @@ -1619,14 +1618,16 @@ void ImFontAtlasBuildSetupFont(ImFontAtlas* atlas, ImFont* font, ImFontConfig* f
  {
  if (!font_config->MergeMode)
  {
  - font->ContainerAtlas = atlas;
  - font->ConfigData = font_config;
  - font->ConfigDataCount = 0;
  + ImVec2 display_offset = font->DisplayOffset;
  +
  + font->Clear();
  +
  font->FontSize = font_config->SizePixels;
  + font->DisplayOffset = display_offset;
  + font->ConfigData = font_config;
  + font->ContainerAtlas = atlas;
  font->Ascent = ascent;
  font->Descent = descent;
  - font->Glyphs.resize(0);
  - font->MetricsTotalSurface = 0;
  }
  font->ConfigDataCount++;
  }
  