Upgrade the Flanger effect

[PhysSong]

Originally authored by @douglasdgi in #5168.
Summary of changes:

1. Allow negative feedback values
2. Add a phase knob to change the stereo LFO phase
3. Move QuadratureLfo class to a single file, include/QuadratureLfo.h

[LmmsBot]

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Linux

• Linux (AppImage): lmms-1.2.3-837%2Bg3dcae02-linux-x86_64.AppImage (build link)

Windows

• Windows 32-bit: lmms-1.2.3-837%2Bg3dcae02d8-mingw-win32.exe (build link)
• Windows 64-bit: lmms-1.2.3-837%2Bg3dcae02d8-mingw-win64.exe (build link)
• Windows 32-bit: lmms-1.3.0-alpha-msvc2017-win32.exe (build link)
• Windows 64-bit: lmms-1.3.0-alpha-msvc2017-win64.exe (build link)

macOS

• macOS : lmms-1.2.3-837%2Bg3dcae02d8-mac10.14.dmg (build link)

🤖

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[PhysSong]

@curlymorphic @douglasdgi FYI.

[curlymorphic]

Looks ok to me, my comments are minor.

[PhysSong]

I squashed commits manually because GitHub's "squash and merge" will make me the primary author whereas it should be @douglasdgi.

[PhysSong]

@DomClark Thanks for your review comments. Pinging @douglasdgi in case he can reply some of them.

[PhysSong]

I think I have to fix them myself, except the last one which I can't find a good reason to fix.

[PhysSong]

Fixed two review comments from @DomClark. I also added the unit for the hint text of the phase knob.

[IanCaio]

@curlymorphic Do you think this PR is ready for merge? You have a valid point on the trigonometric function comment, should it be fixed in this PR or do we add this to a TODO list and check all the plugins for this type of thing? Think the latter would make more sense as we could create a header on the core with those look up tables and replace those functions everywhere at once. If you approve this I'm merging it!

[PhysSong]

I will have to squash commits manually and fast-forward to master because GitHub's "Squash and merge" will mark me as primary author.

[curlymorphic]

the trigonometric function comment, should it be fixed in this PR

Your suggestion of treating the use of the trigonometric functions as a separate issue is a good idea, and would probably lead to a more consistent solution.

I feel this is ready for merge

[IanCaio]

Thanks @curlymorphic ! Think we should open an issue to keep track of that change? I was about to open it but our issue log is so polluted I wonder if we should just remind ourselves and tackle it soon. We'd probably need to make the look-up tables on lmms_math.h then grep every use of sin/cos/tan (and others) and replace them.

@PhysSong Leaving this one for you to merge so you can handle that authorship issue 🚀

[curlymorphic]

Think we should open an issue to keep track of that change? I was about to open it but our issue log is so polluted I wonder if we should just remind ourselves and tackle it soon.

I would agree, that in the scheme of things, optimizing standard functions may not be the best use of LMMS devs time.

[IanCaio]

I would agree, that in the scheme of things, optimizing standard functions may not be the best use of LMMS devs time.

Sorry, I might have expressed myself badly! I think we should optimize those, I just meant that our issue tracker is bloated (could definitely use a huge clean up) so maybe we should just remind ourselves of doing this through Discord. I think it wouldn't be too big of an effort (basically writing the optimized methods on the header and doing a big sed replace. Could be done during the reorg maybe since headers will be moving around.