Add a boolean operator that performs bitwise boolean operations between images

[mhirsch]

This pr adds a boolean operator to sharp that takes a second input image, and performs a selected bitwise boolean operation on a pixel-per-pixel basis between the two images. boolean accepts both files and buffers, much like overlayWith, and uses the same boolean operators as bandbool. It relies on the underlying vips function boolean, which handles casting data types and padding the right-hand operand with zeros to match the size of the left-hand operand.

[lovell]

The rather handy vips_enum_from_nick function should allow this to be simplified:

return static_cast<VipsOperationBoolean>(
  vips_enum_from_nick(nullptr, VIPS_TYPE_OPERATION_BOOLEAN, opStr.data())
);

[lovell]

I've added a few comments inline, thanks for this Matt.

[mhirsch]

I took a crack at the "passing as a vector of buffers" improvement you suggested. I stored the node name and buffer size so its easier to iterate over the vector in the constructor. I think, since the constructor runs in the main thread, we could get away without storing the buffer length, but I thought I would avoid the ambiguity.

[lovell]

If we only append buffers to the vector where their length is >0 then we don't need to store the length.

[mhirsch]

@lovell I tried a couple of methods of finding a pointer that is constant between the constructor and HandleOKCallback, and didn't find something that worked. In 4c7f339 I made the mistake of not passing the buffer by reference, but fixing this, it appears that the iterator object is actually declared on the stack (from the memory address) and differs between range loops while the buffer element is on the heap. I wasn't aware that that's how c++11 range loops worked -- so I learned something. In e2cda35, I expected going to a standard for loop over the length of the std::vector, and getting the address of the vector element to work. However, it seems the referenced object has been copied at some point. I realize there is some ambiguity about which vector is being referenced in the constructor due to the class member saveBuffers having the same name as the constructor argument, but I changed the names and the pointers still differed. I'm not sure if this has to do with some obscure fact of a Local<Object> object, or copying std::vectors but I couldn't find a solution and I'm out of time for tonight. The most recent commit goes back to the BufferContainer method of filling the vector.

I think this is going a little off topic from the boolean operator. Can we merge this one, and then work out a better system for passing in buffers to be saved in a later commit?

I would like to be able to reference this mechanism, whatever it is, in a future commit to pass buffers in for other operators that take images.

[coveralls]

Coverage decreased (-0.1%) to 97.706% when pulling c4f42f0 on mhirsch:boolean into 83d8847 on lovell:master.

[lovell]

Thank you Matt!

[lovell]

I was able to simplify* the SaveTo/GetFromPersistent logic a little in fee3d88 (and e10aeb2 for reasons of d'oh) by using an index-based approach.

*assuming people consider a more functional style to be simpler :-/