Make the dependency on FreeImage optional

[mahiuchun]

Desired behavior

gz-common could be built without using FreeImage.

Alternatives considered

N/A

Implementation suggestion

Use https://github.com/nothings/stb/blob/master/stb_image.h for image loading.

Additional context

• By not supporting less commonly used image formats, the attack surface of image loading is reduced.
• FreeImage has an active SVN but they have not made a release since 2018.

[wentasah]

This might be a good idea, as freeimage is known to contain many security vulnerabilities.

[mjcarroll]

Yes, there has been a little bit of work attempting to replace the dependency with something like stb (#590) but I haven't had bandwidth to continue on it.