Ensure flanger LFO starts at the same position on render

[josh-audio]

Fixes #3689

Here are two GUI renders:

The two renders weren't identical, and that doesn't sit well with me. Regardless, the LFO works exactly the same on both renders.

[PhysSong]

@SecondFlight Good work! I wonder if this could go into stable-1.2.

[josh-audio]

I imagine it could. The merge doesn't work by default of course, but I'm sure I can find a way to do get the commits on 1.2.

If they go on 1.2, is there anything I would need to do to get them on master?

[PhysSong]

If they go on 1.2, is there anything I would need to do to get them on master?

We merge stable-1.2 into master at appropriate time, it's a common workflow.

[curlymorphic]

@SecondFlight nice work,

[josh-audio]

I'm having trouble building 1.2, based on what I'm seeing it looks like I'd have to build Qt 4 from scratch, and I don't really want to do that just yet. I'll still implement the changes since the code should all be the same, but I can't actually test the functionality.

[PhysSong]

Since #4363 is opened, I'll close this one.