bump stb

* stb 057f29f...83c5588 (21):
  > Remove unused variables and ambiguous dangling else
  > Ensure GCC that p is not used uninitialized
  > Remove stb_readdir_size which was written (on windows) but never read
  > Add missing return type declaration
  > Don't read uninitialized memory
  > added a few more FAQ questions
  > tilemap editor 0.30
  > various updates
  > properties and links
  > update version number
  > colorpicker; new color scheme
  > fix bug in paste to protected layer; add eraser tool
  > tiny bit of documentation
  > fix c++ compilation
  > Merge branch 'master' of https://github.com/nothings/stb
  > add stb_tilemap_editor.h
  > version numbers
  > update version number & contributors
  > Merge branch 'master' of http://github.com/rygorous/stb into working
  > Merge branch 'master' into working
  > debugging truetype crash
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