Add Vips::Image#close.

[ioquatix]

I've been using this to help deal with memory usage issues.

[Nakilon]

Hi. Just wondering. It is the thing that you've mentioned here? #67 (comment)
As I understand you said there it didn't help, only the GC.start did or something.

[ioquatix]

Wow that was a while ago.

Well, I have not run the benchmarks again. That being said, in theory a close method makes sense. In practice, more work might be needed to make it practical/useful.

[jcupitt]

Hi, good idea, I've been meaning to add this.

Could we call it invalidate? That's the name we've used in the other libvips bindings.

close is rather close to post_close, the name we use for cleanup-on-finalize handlers.

[jdelStrother]

Doesn't this also need a Vips.attach_function :vips_image_invalidate_all, [:pointer], :void somewhere? Without it I get NoMethodError