fix(macOS): move the pointer once per relative packet

[RodrickOMG]

The libvirtualhid macOS backend performed three conflicting pointer updates per relative motion packet: a CGEvent carrying both an absolute location and kCGMouseEventDeltaX/Y (deltas run through pointer acceleration and were computed from the unclamped position), followed by CGWarpMouseCursorPosition(), which shifts the baseline the next packet reads. Relative motion therefore collapsed to near zero with visible jitter, while button and scroll events -- which are never warped -- kept working. Motion was also clamped to a single display.

Resolve relative motion against the live cursor position and post exactly one event with no warp on top. Re-base on the server position only on the first packet or after a gap: CGEventGetLocation() reports what the window server has already committed, so re-reading it for every packet in a 500-1000 Hz burst makes each packet overwrite the previous one's movement. Accumulate the chain locally in between instead.

Clamp to the full active-display layout, select the event type from the actual button state so drags keep working, and carry the live modifier flags so a drag started while holding Shift/Option/Ctrl keeps them on every motion event that follows the button press. Fall back to the libvirtualhid path when Accessibility is not granted, so behaviour there is unchanged.

Windows and Linux are unaffected.

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[ReenigneArcher]

I think you're trying to fix some of the same things as LizardByte/libvirtualhid#145 and #5739

Can you test #5739 ?

[sonarqubecloud]

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[RodrickOMG]

Thanks for the pointers! I looked into this on my setup (macOS, single display, Moonlight on Windows):

• With "Optimize mouse for remote desktop" enabled, Moonlight sends absolute motion, which goes through libvirtualhid's absolute path rather than the relative path changed in this PR. From its description, chore(deps): bump libvirtualhid #5739 / fix(input): target configured mouse viewport libvirtualhid#145 target the mouse viewport for multi-monitor setups, which doesn't apply to my single-display setup, so I don't think they overlap with the relative-motion issue fixed here.

I haven't tested #5739 yet.