Fix: Split channel overflowing when height:auto

[htc36]

Short description

Resolves #3690

Implementation details

Alter getHeight function to divide the parentHeight by number of channels if the height=auto of the wavesurfer instance

How to test it

• Give the parent container a height
• Set height : auto & splitChannel : [] of Wavesurfer instance
• Observe the waveforms not overflowing outside the parent container as their heights are

Screenshots (Observe split channels is half the size of the parent)

splitChannel=[]	splitChannel=undefined

[katspaugh]

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