Generally, when transforming a difference between two points, `p1`
and `p2`, with a matrix `m`, we would like the following property
to hold:
```
m * (p1 - p2) == m * p1 - m * p2
```
Unfortunately, it wasn't like this before, because matrices have a
translation component, which is non-linear. In `m * p1 - m * p2`,
the translations of `m * p1` and `m * p2` are the same and therefore
cancel out each other. However, in `m * (p1 - p2)` the translation
stays.
In order to preserve this property, introduce a new `PointDelta`
type which is not subject to translation when transformed by a matrix.
For now, the following operations are supported:
* `Point - Point -> PointDelta`
* `Point + PointDelta -> Point`
* `Point += PointDelta`
* `Point - PointDelta -> Point`
* `Point -= PointDelta`
As a consequence, the expression `position + global_to_local_matrix * mouse_delta`
in `update_drag()` now ignores translation, which fixes#817.
* `global_to_local` returns `None` if the object has zero scale.
* Adjust AVM `globalToLocal` methods to return the untransformed
point on failure.
* Add `DisplayObject::mouse_to_local` to handle AVM `mouseX`
and `mouseY` coordinates. For zero scale objects, these end up
returning values based on the twips-to-pixels scale,
divided by 20.
* Add `Matrix::determinant`.
* Rename `Matrix::invert` to `inverse`.
* `Matrix::inverse` return an `Option`, with `None` returned
for non-invertible matrices.
* AMV `Matrix::invert` duplicates the code as the behavior is
different (works in f64 and not twips, etc.)
Nathan Adams