Option to not #define STB_IMAGE_IMPLEMENTATION / STB_IMAGE_WRITE_IMPLEMENTATION

[lmdsp]

As stb_image is quite a popular library, it is quite possible that STB_IMAGE_IMPLEMENTATION and/or STB_IMAGE_WRITE_IMPLEMENTATION macros will already have been set elsewhere, which leads to multiply defined symbols and possibly linker errors.

It would be quite straightforward to modifiy plutovg_surface.c to avoid this, something along the lines of this:

#ifndef PLUTOVG_EXTERNAL_STB_IMAGE_WRITE
#define STB_IMAGE_WRITE_IMPLEMENTATION
#endif // PLUTOVG_EXTERNAL_STB_IMAGE_WRITE

#include "stb_image_write.h"

#ifndef PLUTOVG_EXTERNAL_STB_IMAGE
#define STB_IMAGE_IMPLEMENTATION
#endif // PLUTOVG_EXTERNAL_STB_IMAGE

#include "stb_image.h"

This way we can control wether only stb function prototypes get defined or not and no other part of plutovg is changed.