Enable FreeType bitmap glyphs

[metarutaiga]

This PR is support TTF bitmap for CJK.
We need it because the font is not clear when screen resolution is 1080P or below.
the freetype support bitmap, the stb_truetype don't support it yet.

Before :

Before : (Monochrome)

After :

Example code :

    ImFontConfig font_config;
    font_config.SizePixels = 13.0f;
    font_config.FontBuilderFlags = ImGuiFreeTypeBuilderFlags_Bitmap;
    io.Fonts->AddFontFromFileTTF("C:\\Windows\\Fonts\\msgothic.ttc", 13.0f, &font_config, io.Fonts->GetGlyphRangesJapanese());

[Demonese]

Nice, I'm using WenQuanYi and want to display bitmap glyphs (because of 800x600 LCD screen).
Thank you!

[ocornut]

Merged, thank you!
It's a little unclear to me how this affect sizes (via FT_SIZE_REQUEST_TYPE_NOMINAL vs FT_SIZE_REQUEST_TYPE_REAL_DIM) in different use cases... but at least works very well with msgothic.