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.
3046d68da1