Implement new input API and refactor geometry types

[fwcd]

The implementation of the new input API is analogous to the TypeScript implementation (with some tweaks to make the representation of the events more idiomatic in Rust):

• Implement the new input API nighthouse#12
• Update new gamepad event API to represent diffs nighthouse#13

Additionally, this refactors the geometry types (Pos, Delta and Rect) to be generic over their type and implements them in terms of a common Vec2 type to avoid duplicate implementations of vector arithmetic. This was necessary to represent floating-point mouse positions, as used in MouseEvent, properly.

Since we rename InputEvent to LegacyInputEvent and introduce a new, unrelated type named InputEvent, this constitutes a source break, hence the version bump to 4.0.0 in 9de6ac1.