Update filter histories even when they're not used

If the filter properties are continually updated, and the HF or LF gain goes
from <1, to 1, and later back to <1, the history shouldn't hold stale values
from before it was at 1.

Alc/mixer.c

@@ -260,12 +260,16 @@ static const ALfloat *DoFilters(ALfilterState *lpfilter, ALfilterState *hpfilter
     switch(type)
     {
         case AF_None:
+            ALfilterState_processPassthru(lpfilter, src, numsamples);
+            ALfilterState_processPassthru(hpfilter, src, numsamples);
             break;
 
         case AF_LowPass:
             ALfilterState_process(lpfilter, dst, src, numsamples);
+            ALfilterState_processPassthru(hpfilter, dst, numsamples);
             return dst;
         case AF_HighPass:
+            ALfilterState_processPassthru(lpfilter, src, numsamples);
             ALfilterState_process(hpfilter, dst, src, numsamples);
             return dst;
 

OpenAL32/Include/alFilter.h

@@ -63,6 +63,8 @@ inline ALfloat ALfilterState_processSingle(ALfilterState *filter, ALfloat sample
 
 void ALfilterState_processC(ALfilterState *filter, ALfloat *restrict dst, const ALfloat *src, ALuint numsamples);
 
+void ALfilterState_processPassthru(ALfilterState *filter, const ALfloat *src, ALuint numsamples);
+
 
 typedef struct ALfilter {
     // Filter type (AL_FILTER_NULL, ...)

OpenAL32/alFilter.c

@@ -420,6 +420,24 @@ void ALfilterState_setParams(ALfilterState *filter, ALfilterType type, ALfloat g
     filter->process = ALfilterState_processC;
 }
 
+void ALfilterState_processPassthru(ALfilterState *filter, const ALfloat *src, ALuint numsamples)
+{
+    if(numsamples >= 2)
+    {
+        filter->x[1] = src[numsamples-2];
+        filter->x[0] = src[numsamples-1];
+        filter->y[1] = src[numsamples-2];
+        filter->y[0] = src[numsamples-1];
+    }
+    else if(numsamples == 1)
+    {
+        filter->x[1] = filter->x[0];
+        filter->x[0] = src[0];
+        filter->y[1] = filter->y[0];
+        filter->y[0] = src[0];
+    }
+}
+
 
 static void lp_SetParami(ALfilter *UNUSED(filter), ALCcontext *context, ALenum UNUSED(param), ALint UNUSED(val))
 { SET_ERROR_AND_RETURN(context, AL_INVALID_ENUM); }