Make use of stb_image optional

[tksuoran]

I only use geogram purely for geometry related features and as such I prefer to have minimal build of with without any non-essential features. This is also useful to avoid linking issues with STB image which I already link myself.

[BrunoLevy]

Hello,
I'd rather avoid adding too many options to the CMakeFiles that tend to become more and more complicated over the years and difficult to maintain. Is the presence of stb_image really blocking (besides being "non-aesthetic" if you use it somewhere else)? (normally no symbol is exported from stb_image since we define STB_IMAGE_STATIC and STB_IMAGE_WRITE_STATIC before importing it, do you still have duplicated symbols when you use stb_image in another place ?)