Default parameters

[kkaefer]

I'm wondering why the parameters for Bin are set to default to -1, in particular for x and y. Would a better default be 0?

/cc @bhousel

[kkaefer]

Background is that not setting them to 0 explicitly triggers undefined behavior when dividing by the offset (mapbox/mapbox-gl-native#9509)

[bhousel]

I think I just used -1 as uninitialized because 0,0 is a real coordinate that a bin can be packed into.

[kkaefer]

Okay; are there any use cases for initializing a Bin object without setting its location? If not, I think the parameters shouldn't be optional.

[bhousel]

Bins start out with no location until they are packed. So, external code should never supply a location.
see https://github.com/mapbox/shelf-pack-cpp#batch-packing

Only the internal shelf.alloc method should create bins with a location, see here

I'm not sure the most idiomatic (and performant) C++ way to implement this. It would be great to drop the x and y arguments from the Bin constructor, in order to clear up the confusion.