Include a polyline's width in its bounding box

[redbluevn]

Stacked on #1224. The first commit on this branch belongs to that pull request - a mirrored polyline reporting its bounding box in object coordinates - because this change touches the same two methods and would conflict with it otherwise. Review only the second commit, or take this one after #1224 lands.

The problem

A polyline can carry a width, and it is drawn half on each side of the centre line. LwPolyline.GetBoundingBox and Polyline<T>.GetBoundingBox measure the centre line only:

IEnumerable<XYZ> points = this.Vertices.Any(v => v.Bulge != 0)
    ? this.GetPoints<XYZ>(byte.MaxValue)
    : this.Vertices.Select(v => v.Location.Convert<XYZ>());

return BoundingBox.FromPoints(points.Select(p => toWorld * p));

So an entity that covers a strip is reported as the line down its middle, and a drawing whose outermost geometry is a wide polyline measures smaller than it is.

How it was found

Asking AutoCAD for the extents of a 17.6 MB client drawing and comparing:

AutoCAD   (457057.490, -121881.651) .. (1147751.416, 251826.623)
ACadSharp (457132.493, -121806.650) .. (1147676.421, 251751.625)

Exactly 75 units short on all four sides — and the entity at the edge is one LWPOLYLINE with ConstantWidth = 150.

The fix

Bound each segment on its own, so a taper is paid for only where it occurs:

• a straight segment sweeps the strip between its two offset edges, and the offset goes perpendicular to it. Padding in every direction instead would push the box past the flat end cap, and AutoCAD stops at the cap;
• a segment with a bulge is padded by the larger of its two half widths, which contains the arc's sweep;
• a bulge over a segment of no length has no arc (Arc.CreateFromBulge refuses the zero radius), so it contributes the point it sits on, widened.

The widened area is measured in the polyline's own plane and taken to the world afterwards, so a mirrored polyline still lands on the correct side. ConstantWidth (code 43) applies wherever a vertex carries none of its own; for the old-style POLYLINE the polyline's own start and end width (40 and 41) play that role. A 3D polyline carries the fields but no width and is untouched.

Measured against AutoCAD, on drawings it minted itself

fixture	AutoCAD	before	after
closed square, ConstantWidth 150	(-75,-75)..(1075,1075)	(0,0)..(1000,1000)	exact
open PLINE, width 150	(0,-75)..(1075,1000)	(0,0)..(1000,1000)	exact
open, widths tapering 0-200-400	(0,-108.9)..(1200,1000)	(0,0)..(1000,1000)	(0,-100)..(1200,1000)

What is left in the third row is the fill AutoCAD adds where two segments meet, which reaches a little further than either segment does.

On the drawing that prompted this, the extents now match AutoCAD to 3e-06. On nineteen other client drawings not one extent moves, and a polyline with no width keeps exactly the path it had.

Tests

GetBoundingBoxIncludesAConstantWidth, GetBoundingBoxCarriesAWidthAcrossTheSegmentAndNotBeyondItsEnds, GetBoundingBoxKeepsTheCentreLineWithoutAWidth, and GetBoundingBoxIncludesTheWidthTheVertexesCarry for the old-style polyline. Suite 2900 / 0.