X11: EventMouseScroll and EventMouseButton report incorrect cursor position

[navaratine]

These events report incorrect, rarely updated _x and _y values.

I've been able to track down the source of this issue: unlike MotionNotify handler of XI_Motion doesnt update lastMousePosX and lastMousePosY before dispatching EventMouseMove. Handler of MotionNotify does set these values but I find that it's very rarely invoked. This results in scroll and click events often reporting incorrect mouse position.

This can be fixed by replicating behaviour of MotionNotify handler and updating last mouse position before event dispatch.
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[navaratine]

I've opened a PR with the proposed fix. Feel free to merge it if it looks alright to You.