Upgraded stb_truetype.h.

Changelog:
  0.8  (2014-05-25) fix a few more warnings
Upgraded stb_image.h

This adds jpg, bmp, gif, psd, pic, tga support since there's
no longer any way of disabling those.

Changelog:
      1.38 (2014-06-06) suppress MSVC run-time warnings, fix accidental rename of 'skip'
      1.37 (2014-06-04) remove duplicate typedef
      1.36 (2014-06-03) converted to header file, allow reading incorrect iphoned-images without iphone flag
      1.35 (2014-05-27) warnings, bugfixes, TGA optimization, etc
      1.34 (unknown   ) warning fix
Added two internal parser methods to the API.