ImFontAtlas: Rewrote stb_truetype based builder.

- Atlas width is now properly based on total surface rather than glyph count (unless overridden with TexDesiredWidth).
- Fixed atlas builder so missing glyphs won't influence the atlas texture width. (#2233)
- Fixed atlas builder so duplicate glyphs (when merging fonts) won't be included in the rasterized atlas.

docs/TODO.txt

@@ -235,9 +235,7 @@ It's mostly a bunch of personal notes, probably incomplete. Feel free to query i
  - pie menus patterns (#434)
  - markup: simple markup language for color change? (#902)
 
-!- font: need handling of missing glyphs by not packing/rasterizing glyph 0 of a given font.
- - font: MergeMode: flags to select overwriting or not.
- - font: MergeMode: duplicate glyphs are stored in the atlas texture which is suboptimal.
+ - font: MergeMode: flags to select overwriting or not (this is now very easy with refactored ImFontAtlasBuildWithStbTruetype)
  - font: free the Alpha buffer if user only requested RGBA.
 !- font: better CalcTextSizeA() API, at least for simple use cases. current one is horrible (perhaps have simple vs extended versions).
  - font: a CalcTextHeight() helper could run faster than CalcTextSize().y
@@ -252,7 +250,6 @@ It's mostly a bunch of personal notes, probably incomplete. Feel free to query i
  - font/draw: vertical and/or rotated text renderer (#705) - vertical is easier clipping wise
  - font/draw: need to be able to specify wrap start position.
  - font/draw: better reserve policy for large horizontal block of text (shouldn't reserve for all clipped lines)
- - font: imgui_freetype.h alternative renderer (#618)
  - font: optimization: for monospace font (like the default one) we can trim IndexXAdvance as long as trailing value is == FallbackXAdvance (need to make sure TAB is still correct).
  - font: add support for kerning, probably optional. A) perhaps default to (32..128)^2 matrix ~ 9K entries = 36KB, then hash for non-ascii?. B) or sparse lookup into per-char list?
  - font: add a simpler CalcTextSizeA() api? current one ok but not welcome if user needs to call it directly (without going through ImGui::CalcTextSize)