Add ExpDecay function for framerate independent damping

[tebjan]

Is your feature request related to a problem? Please describe.
See, Lerp smoothing is broken, click the video below:

Describe the solution you'd like

instead of:
a = lerp(a, b, damping * dt)

Implementations for both, C# and SDSL.

Here are some questions that can aid in the description of the feature request:

• What are the use cases?
  Daping between two values over time for smooth motion

[Kryptos-FR]

The only issue is that exp() is more expensive computationally.

[tebjan]

The more important issue ist that Lerp smoothing doesn't work correctly. Watch the first half of the video. Especially if you want to deliver a consistent user experience on all machines. It might not affect SFX too badly, but if gameplay relies on it, no option has better performance.