[animator] channel animation playback hangs at the last channel when computed polarization component is selected

[kswang1029]

Describe the bug
When we load a Stokes cube, we can have infinite loop of channel playback for a give Stokes component. However, if a computed polarization component is selected, such as Plinear, the channel playback hangs at the last channel.

To Reproduce
Steps to reproduce the behavior:

1. load a Stokes cube
2. select Stoke Q and trigger channel animation playback. When the last channel is rendered, the next would be the first channel.
3. switch to Plinear or other computed component and select a channel close to the last channel (just to save time for observing)
4. trigger channel animation playback and it will hang at the last channel

Expected behavior
channel animation playback for computed polarization component should behave just like regular Stokes.

Screenshots or videos

Screen.Recording.2022-11-23.at.13.42.25.mov

Platform info (please complete the following information):

• OS [e.g. macOS Monterey]: macOS Monterey
• Browser [e.g. chrome, safari, electron app]: v3-stable electron app
• Browser version [e.g. 22]:
• Backend branch [e.g. dev, v3b2 release]: v3-stable
• Frontend branch [e.g. dev, v3b2 release]: v3-stable

[kswang1029]

Thanks to @crocka for diagnosing this issue. This seems a backend one.

[markccchiang]

Actually, I think this problem is from the incorrect stokes index setting in the StartAnimation's sub-messages AnimationFrame, which is sent by the frontend. I will fix it on the frontend side.