Fixing an issue caused by timeline::GetClip returning a ClipBase* instead of Clip*

[jonoomph]

Fixing an issue caused by timeline::GetClip returning a ClipBase instead of a Clip (broke waveform generation).
Also adding a swig definition for the Caption effect.

[codecov]

Codecov Report

Merging #595 (ac73bd9) into develop (9a91abc) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop     #595   +/-   ##
========================================
  Coverage    52.54%   52.54%           
========================================
  Files          129      129           
  Lines        10707    10707           
========================================
  Hits          5626     5626           
  Misses        5081     5081           

Impacted Files	Coverage Δ
src/Timeline.h	63.63% <ø> (ø)
src/Timeline.cpp	45.41% <100.00%> (ø)

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[ferdnyc]

@jonoomph Gah! Should've realized I was accidentally slicing the Clip object there, thanks for fixing that up.

GetClipEffect() similarly returns EffectBase*, but I suppose there's no helping that. But by the same token, it shouldn't be a problem because the common API all lives there.

[ferdnyc]

I'm surprised the unit tests didn't need changing, I guess I must've used auto for the types. Go me?

Though, a unit test that tries to access the Clip API of the returned object (such as Clip::Waveform) would've caught this, and would probably be a good thing to add to Clip_Tests.cpp Timeline_Tests.cpp.

[ferdnyc]

Defense Against the Dark Slice invocations added to the Timeline unit tests, in #596.