Add support for vips_image_write_to_memory

[versi-tech]

Introducing vips_image_write_to_memory support.

Adjusted logic of adding object_unref calls for return types of void* (like with vips_image_write_to_memory). This should not be directly replaced by g_free as memory ownership is not always the same with different API functions of libvips and should be dealt with externally by the client.

[lopcode]

Thanks again @versi-tech - I took a look at this today, and tweaked it slightly:

• I realised it's possible to use the MemorySegment API to avoid doing a second copy to/from a ByteBuffer, and I think exposing the FFM API in these advanced functions is OK
• Rather than free-ing up front, we can use reinterpret to couple the resulting memory to the lifetime of the arena, which is consistent with how all the other operations work
• Tweaked the VImage.newFromMemory API to also take a MemorySegment rather than byte[] which allows for the same copying optimisation on the other end, and added a VImageFromMemoryToMemory sample to show how that works