EffectComposer: Clock instead of Date

[sciecode]

Should be more accurate than what we had before, for cases where performance is available.

[sciecode]

@mrdoob just clarifying.
I double checked if this._previousFrameTime wasn't being used anywhere else. It is not.
You can see that all examples are still being rendered correctly: postprocessing examples.

Missed TS file

[sciecode]

@Mugen87 would you please confirm this is correct? I'm not too familiar with TS.

[mrdoob]

Thanks!