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`.