Add target write to rawsave

[jcupitt]

Discussed in #3940

^{Originally posted by akash-akya April 18, 2024}
Does it makes sense to create VipsForeignSaveRawTarget to support streaming raw pixels in a unified way (vips_image_write_to_target(in, ".raw", target))?
I know there is already VipsForeignSaveRawFd which can be used to create similar interface, but in our case we have a unified interface to do streaming writes and this would help us to avoid creating special case to write raw pixels.

If this makes sense then I can try implementing it

[jcupitt]

From the discussion:

You can copy-paste code from something like ppmsave. I would have an abstract base class VipsForeignSaveRaw which writes to a target, then define VipsForeignSaveRawTarget, VipsForeignSaveRawFile etc. as concrete subclasses which define the parameters.

[akash-akya]

Thanks for the pointers. I'll try have a PR ready over the weekend.