ImFontAtlas: Rewrote FreeType based builder. ...

- Fixed abnormally high atlas height. (#618)
- Fixed support for any values of TexGlyphPadding (not just only 1). (#618)
- Atlas width is now properly based on total surface rather than glyph count (unless overridden with TexDesiredWidth). (#618)
- Fixed atlas builder so missing glyphs won't influence the atlas texture width. (#2233, #618)
- Fixed atlas builder so duplicate glyphs (when merging fonts) won't be included in the rasterized atlas. (#618)

imgui_freetype: Minor tweaks and comments.

Internals: Replace unsigned short with ImWchar when dealing with character storage (missing cases) + in imgui_impl_ file keep using neutral type everywhere, added missing explicit cast in three bindings. (#2078)

Renamed stb_truetype.h to imstb_truetype.h, stb_textedit.h to imstb_textedit.h, and stb_rect_pack.h to imstb_rectpack.h. (#1718, #2036) ...

If you were conveniently using the imgui copy of those STB headers in your project, you will have to update your include paths.
The reason for this change is to avoid conflicts for projects that may also be importing their own copy of the STB libraries. Note that imgui's copy of stb_textedit.h is modified.

ImFontConfig: Added GlyphMinAdvanceX/GlyphMaxAdvanceX settings useful to make a font appears monospaced, particularly useful for icon fonts. (#1869)

imgui_freetype: skip missing glyphs as well to match our stb loader (#1671, #1703)

Misc comments, todos + reintroduced removed ImGuiCol defines under !IMGUI_DISABLE_OBSOLETE_FUNCTIONS wrap.

ImFont: Added FindGlyphNoFallback. Fixed MergeMode broken by 1ef1acb

Font: Fixed MergeMode adding duplicate glyphs data instead of reusing existing (broken by 072d6d8)

ImFontAtlas: Added ImFontAtlasFlags_NoPowerOfTwoHeight, ImFontAtlasFlags_NoMouseCursors flags. (#1613)

imgui_freetype: comments about correct blending and sRGB (#618, #578)

imgui_freetype: Documentation, tweaks. (#618)

Moved imgui_freetype from imgui_club (#618)