player/video: rewrite display-sync audio drift compensation

The old code had several interacting problems that prevented A/V from
reaching a steady state.

When the A/V difference was large, it applied the maximum allowed
compensation in one step. That was fine for catching up, but the value
was never re-evaluated, so audio speed routinely overshot the target.
Once the difference crossed zero (or re-entered the acceptable range) it
would run a linear regression over recent av_diff samples and snap the
audio speed to the estimated drift. The fatal flaw was that those
samples already included the controller's own compensation: even when
the underlying drift was zero, the controller would "detect" the drift
it had just induced and try to cancel it. Between these rare, sparse
updates audio speed stayed frozen, so in practice the controller was
introducing drift more than it was fixing it. On high fps outputs,
where the vsync range is tight, this degenerated into a ping-pong:
overshoot, snap back, overshoot again, never settling.

The new code replaces the bang-bang direction controller and one-shot
regression with a proportional controller driven by a low-pass-filtered
av_diff. The LP filter (1 s time constant) rejects per-frame vsync
jitter so the compensation no longer chases noise. The target then
decays av_diff exponentially toward zero with a 10 s recovery time.
Adaptive slew caps per-frame motion to the desired correction magnitude,
so large excursions are absorbed quickly while near-equilibrium motion
stays below audibility. The result is a continuous, gentle control loop
that actually reaches steady state and does so without audible
audio-speed modulation.