Freeciv

Focus unit selection animation has intentionally been left out from #1107 for not messing with existing animations. This ticket remains for resolving the issue for focus unit selection animation.

Proposed solution isn't really compatible with our general animations framework. Like #1107 already does for other animations, we should have constant animation refresh rate and animations just defining how many refreshes they miss. This requires only one timer for animations. The proposed solution would mean separate timer for each animation, as they would trigger at different ms intervals.

Typical animation frameworks (eg gtk_widget_add_tick_callback, requestAnimationFrame, QAnimation) are driven by the display driver at the display refresh frequency. Animation progress is derived from a steady clock (as current_time % frame_rate) instead of a timer. This is much less sensitive to system load than a timer-based approach, allows dropping frames when needed, and can be automatically throttled when the window goes out of sight. The principle is also readily extensible to animations running at different frame rates.