tweak(drawable): Decouple stealth detected opacity fade time step from render update

[xezon]

• Fixes Enemy stealthed units/buildings reveal animation flicker or animate too quickly at higher render FPS #1719
• Related to tweak(drawable): Decouple material opacity of detected stealth models from render update #1963

This change decouples the stealth opacity fade time step from the render update. This way it always fades with the same speed regardless of frame rate.

TODO

• Replicate in Generals

[Caball009]

Comment and PR title should clarify that this is about detected stealth models.

[Skyaero42]

Nice!

[xezon]

Replicated in Generals with conflicts.