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:

• [x] Investigate if the maximum smoothing range of 8*FLUID_BUFSIZE can be shortened
• [x] Watch out for performance optimizations (e.g. writing and re-reading the filter coeffs should not go via global memory)
• [x] Publish rendered audio samples for comparison (native 2.4.0 release + 2.4.0 with #1345 reverted + changes as of this PR) - see my comment here for the test tunes: https://github.com/FluidSynth/fluidsynth/issues/1417#issuecomment-2495454180

[sonarcloud[bot]]

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

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