Add convenience methods to correct for non-Z-axis entity normals.

[brettfo]

E.g., Commonly AutoCAD will save a CIRCLE or ARC with a negative Z-axis normal vector which negates the X coordinate of the center and can also cause problems with the start/end angles.

There should be some sort of method (exact shape TBD) that does the following:

public static DxfEntity CorrectForExtrusionVector(this DxfEntity entity)
{
    if (entity.Normal != DxfVector.ZAxis)
    {
        switch (entity.EntityType)
        {
            case DxfEntityType.Circle:
                // TODO: make the necessary transformations to correct for normal and return a copy
            ...
        }
    }

    return entity; // no change
}

Alternately, the individual values could be handled:

public static DxfPoint CorrectForExtrusionVector(this DxfEntity entity, DxfPoint point)
{
    if (entity.Normal != DxfVector.ZAxis)
    {
        // TODO: transform the point as appropriate
    }

    return point; // no change
}

[uzername]

I encounter that sometimes too. Especially it was noticeable for arcs. Transformation steps for me are:

1. negate X coordinate of arc center ( ArcCenterX = -ArcCenterX )
2. Horizontally mirror start angle and end angle
3. Swap values between start angle and end angle