Add movement threshold before activating viewport rotation gizmo dragging

[ryevdokimov]

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~~Requires and Includes: https://github.com/godotengine/godot/pull/101369~~ (This was merged, and this PR was rebased).

This gets activated too quickly, and sometimes accidentally activates when trying to select an axis. This PR adds a movement threshold before activating.

LMB is being held down at the start of the video:

https://github.com/user-attachments/assets/766bc1e5-fc1e-41ee-a48a-3637be2e93e9