Add delta to CursorMoved event

[lynn-lumen]

Objective

• Fixes #11695

Solution

• Added delta: Option<Vec2> to bevy_window::CursorMoved. delta is an Option because the CursorMoved event does get fired when the cursor was outside the window area in the last frame. In that case there is no cursor position from the last frame to compare with the current cursor position.

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Changelog

• Added delta: Option<Vec2> to bevy_window::CursorMoved.

Migration Guide

• You need to add delta to any manually created CursorMoved struct.

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