Fix modLfoToVol behavior

[derselbst]

Previously, modLfoToVol never caused the volume to increase, because amplitude calculation is performed in fluid_cb2amp() which previously forbids negative values. This PR changes this behavior and permits negative values. I believe this is a rare case, hence I'm calculating the values on the fly rather than using lookup tables as we usually do in such cases

Fixes #1374.

[derselbst]

@mrbumpy409 FYI. Seems like test case column nr. 5 should have been red previously ;)

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[mrbumpy409]

@mrbumpy409 FYI. Seems like test case column nr. 5 should have been red previously ;)

Good point, haha! At the time, I saw the same volume clamping with Audigy 2 when the track volume was set to 127, so I figured FluidSynth was emulating that behavior.

With CC7 = 127 and velocity = 127, the modLfoToVol test on Audigy 2 ZS looks like this (note, my newer, yet-to-be-published version of this test uses a sine instead of saw wave):

Anyway, I actually understand the SoundFont 0dB voice volume ceiling now, and future versions of my test will properly account for that.