fix(vm): x87 native filter unit was denormalizing and eating up CPU

When voice was silent, the exponential decays in the filter unit
were causing the high pass component to eventually denormalize,
causing high CPU loads. The solution is the same as in the delay
unit: add and subtract a small number from the value, causing
essentially a flush to zero.
https://en.wikipedia.org/wiki/Subnormal_number

Fixes #68.

vm/compiler/templates/amd64-386/effects.asm

@@ -156,6 +156,9 @@ su_op_dbgain_mono:
     fsubp   st2, st0                        ; r x-l'
     fmul    dword [{{.WRK}}+8]  ; r*b x-l'
     fsubp   st1, st0                        ; x-l'-r*b
+    {{- .Float 0.5 | .Prepare | indent 4}}
+    fadd    dword [{{.Float 0.5 | .Use}}]           ; add and sub small offset to prevent denormalization
+    fsub    dword [{{.Float 0.5 | .Use}}]           ; See for example: https://stackoverflow.com/questions/36781881/why-denormalized-floats-are-so-much-slower-than-other-floats-from-hardware-arch
     fst     dword [{{.WRK}}+4]  ; h'=x-l'-r*b
     fmul    dword [{{.WRK}}+12]                   ; f2*h'
     fadd    dword [{{.WRK}}+8]  ; f2*h'+b