When compiling with -O3, gcc would complain that 'linear' might not be
initialized if comp is superior to 4.

In fact passing a value > 4 is an error anyway, but gcc does not know
that.  I changed the switch case to support comp > 4.  I don't think it
should affect the performances.

rename all STBI_ config macros to STBIW_ to avoid conflict with stb_image.h

Fixes #90.

also updated the comment at the top of the file to reflect stbi_write_tga's new capabilities.

rename stb_image_write private functions to use stbiw__ not stbi__ to avoid conflict with stb_image.h