Fix for warning 4244. Cast to short

Using stb_image.h. Compiling with Visual Studio 2013, with /W4 and getting:

`stb\stb_image.h(1771) : warning C4244: '=' : conversion from 'int' to 'short', possible loss of data`

`s` is a 4 bit value, so just added a cast.
(My editor trimmed some trailing whitespace)