MultirotorMixer: hotfix [-1,1] scaling

[MaEtUgR]

Describe problem solved by this pull request
Taking off with a powerful enough multirotor which has hover thrust < 50% leads to a vertical flyaway. E.g. SITL jmavsim
By bisecting I found that #15563 introduced that and after inspection I found that only the idle thrust part has enough influence to make that happen. Apparently the output was scaled with [0,1] instead of [-1,1].

Describe your solution
The variable _idle_speed which was removed in #15563 was always -1.f so I just followed where it was used before and replaced the previous use with the number.

Describe possible alternatives
We should probably reconsider suddenly changing the output scaling in the middle of the mixer. This is just a hotfix to restore previous functionality without _idle_speed.

Test data / coverage
SITL flying works again 🎉

Additional context
Bug is not on v1.11

[MaEtUgR]

Something seems still not good. The mixer test fails. Let me check.

[dagar]

I suppose it's a little late now, but can we also add something simple to the mixer unit tests that would have caught this in the first place?

[dagar]

Depending on the constructor (geometry or actual scaling) you get a different idle speed.

https://github.com/PX4/Firmware/blob/0ff4fad95cfb8bc4a2fa62088363e57b7c037b23/src/lib/mixer/MultirotorMixer/MultirotorMixer.cpp#L82-L103

[MaEtUgR]

Depending on the constructor (geometry or actual scaling) you get a different idle speed.

That's also what I had found out. Default value for _idle_speed was 0.f but the output_limit_calc function for all mixer output wants [-1,1]. So without changing all the output modules and mixers to a different scaling it was easier to have a rescale adaption in the mixer test. We can reconsider how scaling should be but as a hotfix I consider this the nicest solution.

[MaEtUgR]

For CI: #12148 (comment)

[dagar]

Thanks for the quick fix @MaEtUgR, apparently this wasn't such a quick and easy flash savings after all...