[PD] iem filter interpolation

[Tim Blechmann]

> > maybe the usage of block~ with a blocksize-argument less than 64
> well, a blocksize of 4 was enough for my ears ...
> still, profiling i figured out, you are using sin/cos instead of
> sinf/cosf, which should be faster ... since they work with float
> instead of doubles ... not part of ansi c but of c99 (at least gcc
> supports them)...

hm ... doing even more profiling on hml_shelf~, i'm found that the
stability checking (at a blocksize of 4) is much more expensive than the
actual _perform function, due to the sinf, cosf calls...

the specific part from opreport:
111961    2.3181  libm-2.3.4.so            __sinf
107115    2.2177  hml_shelf~.pd_linux      sighml_shelf_calc
103545    2.1438  hml_shelf~.pd_linux      sighml_shelf_perform
54676     1.1320  libm-2.3.4.so            __cosf

i'm not really familiar with filter design, but hot accurate does the
sin/cos functions have to be? would it make sense to replace the libm
functions with table lookups (like in osc~)? not sure, if it would be
accurate enough, though ...

cheers ... tim

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