fix MSVC-ARM internal compiler error by wrapping malloc

For some reason Microsoft CL compiler for ARM is unable to compile malloc when parameter is an expression. malloc(x * y) will cause internal compiler error, malloc(x) is however fine.
stbi_write_tga can now write 1 and 2 channel (grayscale and grayscale-alpha) images.
comment was wrong way around... code was right.
realized the 2 colorbytes case is impossible.
also updated the comment at the top of the file to reflect stbi_write_tga's new capabilities.
Merge branch 'master' of https://github.com/thedmd/stb into working

Conflicts:
	stb_image.h
Merge branch 'master' of https://github.com/Skylark13/stb into working
tweak TGA monochrome support
fix warnings
fix broken map generation
update version numbers