Better bbox reporting

[springmeyer]

We should implement @brandonreavis's suggestions at:
mapbox/mapnik-omnivore#43 (comment)

mapnik_projection.cpp

double minx = a->Get(0)->NumberValue();
double miny = a->Get(1)->NumberValue();
double maxx = a->Get(2)->NumberValue();
double maxy = a->Get(3)->NumberValue();
p->projection_->forward(minx,miny);
p->projection_->forward(maxx,maxy);
Local<Array> arr = NanNew<Array>(4);
arr->Set(0, NanNew(minx));
arr->Set(1, NanNew(miny));
arr->Set(2, NanNew(maxx));
arr->Set(3, NanNew(maxy));
NanReturnValue(arr);

should become something like this:

double ulx, ulx, urx, ury, lrx, lry, llx, lly;
ulx = llx = a->Get(0)->NumberValue();
lry = lly = a->Get(1)->NumberValue();
lrx = urx = a->Get(2)->NumberValue();
uly = ury = a->Get(3)->NumberValue();
p->projection_->forward(ulx,uly);
p->projection_->forward(urx,ury);
p->projection_->forward(lrx,lry);
p->projection_->forward(llx,lly);
Local<Array> arr = NanNew<Array>(4);
arr->Set(0, NanNew(ulx<llx ? ulx : llx));
arr->Set(1, NanNew(lry<lly ? lry : lly));
arr->Set(2, NanNew(urx>lrx ? urx : lrx));
arr->Set(3, NanNew(ury>uly ? ury : uly));
NanReturnValue(arr);

[flippmoke]

@springmeyer just to be clear on the problem here, I wanted to draw what the current code does, and then consider what we should do going forward.

If you reprojected epsg:4326 to epsg:3857 as a box in the northern hemisphere you might get a resulting box like the dashed blue area shown. The red points are the points we use from a currently transformed bounding box and the green is the new bounding box we form.

Our current implementation is bad, so we should switch to what he is suggesting, however, this MIGHT also be confusing to some people because you have the possibility to include data that you didn't expect to include in the first place with bounding boxes like this.

The fool proof solution would be track all 4 corners of a bounding box always, but this would complicate so much.

/cc @artemp

[flippmoke]

Implemented fix in 90de0d6 and 3e24c5a