Allow to use vips_block_untrusted_set and vips_operation_block_set methods

[aglushkov]

Some forgotten stuff I think...

I wrote tests same way as they are in python library
https://github.com/libvips/pyvips/blob/a9688b9c7c2a4b448eec0f0a9375c8f34e76354e/tests/test_block.py#L38

[jcupitt]

Hi @aglushkov,

Thank you for doing this work, and sorry for sitting on it for so long.

      Vips.vips_block_untrusted_set(true)

This is a little ugly. How about making a tiny wrapper, like the one for vips_vector_set_enabled()?

https://github.com/libvips/ruby-vips/blob/master/lib/vips.rb#L703-L708

So eg. maybe:

  # Block untrusted loaders from executing.
  def self.block_untrusted enabled
    vips_block_untrusted_set(enabled ? 1 : 0)
  end

  # Set or clear the block flag on a named operation.
  def self.block name
    vips_operation_block_set(name)
  end

[aglushkov]

@jcupitt No worries, added commit with wrappers
Not sure if it's ok to have second enabled parameter, or you want a separate unblock methods.
Also not sure its correct place for wrappers and attach_functions in operation.rb file.
And I would like to check if provided operation name exists, but don't know how I can do this easily

[jcupitt]

We have the global Vips stuff in lib/vips.rb, the wrappers should probably go in there.

Ah you're right, I forgot the enabled param to vips_operation_block_set(). It should probably be required and not optional.

[jcupitt]

... could you also add a line to CHANGELOG and credit yourself?

[jcupitt]

This looks really nice. Thank you for adding this feature @aglushkov !