Support blocking untrusted operations

[atjn]

Support blocking untrusted and arbitrary operations with vips_block_untrusted_set and vips_operation_block_set:

// Block all untrusted operations
vips.blockUntrusted(true);

// Block all foreign loaders
vips.operationBlock("VipsForeignLoad", true);

// Reenable the jpeg loader
vips.operationBlock("VipsForeignLoadJpeg", true);

Note that it was already possible to use vips_block_untrusted_set before, by setting the module.ENV.VIPS_BLOCK_UNTRUSTED parameter on initialization.

Related: #21 (comment)

[atjn]

I considered integrating this a bit deeper, allowing for things like:

vips.Operation.block(vips.Image.pngsave, true);

vips.Image.jpegsave.block(true);

vips.Image.jpegsave.block(); // returns true
vips.Image.jpegsave.untrusted(); // returns false

But AFAICT it would require a massive amount of work, and I wasn't even sure if it would be desirable, given that it is pretty different from the core libvips API.

[kleisauke]

Thanks for working on this! I left a couple of comments inline.

[kleisauke]

Thanks all! This will be in v0.0.4.