[2/2] Preview animated video speed

[dazzatronus]

Without this, authors can't see a speed ramp before they render. The preview now plays a video's animated speed the way renders do. The source starts at trim and moves by the integrated speed curve (trapezoid rule, 0.01 s steps). The first and last speeds hold, and gaps blend linearly.

• Out-of-range rates: when the instantaneous speed is outside the browser's playbackRate range (holds, reverse, above 16×), the player pauses and seeks each tick instead. This also stops constant speeds below 0.0625 from throwing.
• Speed control: it hides for animated speed, so it can't flatten the curve.
• Non-number speeds: a non-number speed, including an empty array, previews at 1×.

Verify: npm test. There are new cases in player-sync, media-player-fallback and media-toolbar.

#	PR	Adds	Why separate
1	#208 Bump @shotstack/schemas to 1.20.0	Schema pin	Dependency bump. Merge together with #209
2	#209 Preview animated video speed (this PR)	Preview support	The behaviour change

Code:

• createAnimatedSpeed
• getSourceTime
• rate guard
• getAssetSpeed
• speed control visibility