ImFontGlyphRangesBuilder: Using 32-bits fields for storage instead of 8-bit ones, comments, todo.

docs/TODO.txt

@@ -319,6 +319,7 @@ It's mostly a bunch of personal notes, probably incomplete. Feel free to query i
  - optimization: replace vsnprintf with stb_printf? or enable the defines/infrastructure to allow it (#1038)
  - optimization: add clipping for multi-component widgets (SliderFloatX, ColorEditX, etc.). one problem is that nav branch can't easily clip parent group when there is a move request.
  - optimization: add a flag to disable most of rendering, for the case where the user expect to skip it (#335)
+ - optimization: fully covered window (covered by another with non-translucent bg + WindowRounding worth of padding) may want to clip rendering.
  - optimization: use another hash function than crc32, e.g. FNV1a
  - optimization/render: merge command-lists with same clip-rect into one even if they aren't sequential? (as long as in-between clip rectangle don't overlap)?
  - optimization: turn some the various stack vectors into statically-sized arrays