BR: Rendering clip and multisampler do not work when clip is on a child of a submix

[Robert Randolph]

The render clip function, and the clip multisampler function do not work for me when the clip is on a track that is the child of a submix track.

To reproduce:

• Create a track
• Add a clip to the track
• Use the 'Create submix track containing...' function
• Attempt to drag the 'render clip' icon on the clip. This fails for me.
• Attempt to use the clip multisampler function. This fails for me.
• Remove the submix, and now both functions work.

There may be other functionality broken, but these are the 2 things I noticed.