Port DSP code to C++ to address performance penalty

[derselbst]

I found that after merging #1444 a small performance penalty was introduced. Looking at VTune's profiling report (see below) showed the outer if clause of fluid_iir_filter_apply being pretty busy. I assume this is because the calling code in fluid_rvoice_dsp.c invokes that function twice, once with the custom filter typically being disabled, and the regular low pass filter being enabled. Before merging #1444 this was done once per voice. Now it's done for every single sample, which apparently hurts branch prediction and possibly also instruction caching.

To address this, I ported the DSP code to C++98, which allows me to use templating to generate multiple versions of the DSP code. One of them e.g. never attempts to use the custom IIR filter in the DSP loop if it's unused for that voice. Also whether or not to construct a 24bit sample has been templatified.

In addition, most of the IIR filter code was moved to the header, allowing the compiler to inline it.

With these changes, our testsuite make check_manual now finishes up 7 seconds faster for me.

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Profiling before this PR (at rev e57360d):

Profiling after this PR, fluid_iir_filter_apply is no longer that busy.

[sonarqubecloud]

Quality Gate failed

Failed conditions
19.1% Duplication on New Code (required ≤ 5%)

See analysis details on SonarQube Cloud