Smooth linear filter parameter change

[derselbst]

This PR implements a smooth change of the IIR filter parameters fres (=filter cutoff frequency) and Q. It is meant as a proper replacement for the previous linear smoothing of the individual filter coefficients, see #1345.

This PR is supposed to fix the clicks and pops heard in those related issues:

Fixes #1415
Fixes #1417
Fixes #1424

PR Depends on:
#1429
#1430

Open TODOs:

• Investigate if the maximum smoothing range of 8*FLUID_BUFSIZE can be shortened
• Watch out for performance optimizations (e.g. writing and re-reading the filter coeffs should not go via global memory)
• Publish rendered audio samples for comparison (native 2.4.0 release + 2.4.0 with Remove interpolation of IIR filter coefficients #1345 reverted + changes as of this PR) - see my comment here for the test tunes: strange audio distortion when using filter envelope after merging #1345 #1417 (comment)

[sonarcloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
63.2% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[spessasus]

LGTM, should release this ASAP as it fixes modEnv filter which I consider a major (commonly used) SF2 feature.

[derselbst]

Thanks for the feedback, I'm on it.