And make it generic, as a first step towards making it a general-purpose
data structure for the whole codebase. Some potential replacements are:
* `BoundingBox` in `render/src/bounding_box.rs`.
* `BoxBounds` in `core/src/html/dimensions.rs`.
* Parameters to a bunch of `BitmapData` methods in
`core/src/bitmap/bitmap_data.rs`.
Fixes some issues with our winding # calculation which would cause
incorrect results for hitTest.
* The convention for handling an intersection at endpoints was
not the same between lines and bezier curves.
* The bezier curve winding # function was not properly handling
some cases where the curve was strictly y-monotonic.
* Simplify the code a bit so that ray-curve intersections are
returned in a consistent order based on upward/downward crossing.
Nathan Adams