Make fallthroughs explicit in stb_image.h

[r-barnes]

The -Wimplicit-fallthrough flag can be used to check for implicit fallthroughs, which are a common source of errors.

I've added and used STBI_FALLTHROUGH which maps to __attribute__((fallthrough)), which the compiler uses to determine whether a fallthrough is explicit or not.

[nothings]

This needs to work on all compilers, so the define needs to be an ifdef which tests for suitable compilers, with an else that defines it to nothing.