audio: make the resampling code greedy

Read the maximum possible number of audio frames instead of the
minimum necessary number of frames when the audio stream is
downsampled and the output buffer is limited. This makes the
function symmetrical to upsampling when the input buffer is
limited. The maximum possible number of frames is written here.

With this change it's easier to calculate the exact number of
audio frames the resample function will read or write. These two
functions will be introduced later.

Acked-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Acked-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Volker Rümelin <vr_qemu@t-online.de>
Message-Id: <20230224190555.7409-3-vr_qemu@t-online.de>

audio/rate_template.h

@@ -40,8 +40,6 @@ void NAME (void *opaque, struct st_sample *ibuf, struct st_sample *obuf,
     int64_t t;
 #endif
 
-    ilast = rate->ilast;
-
     istart = ibuf;
     iend = ibuf + *isamp;
 
@@ -59,15 +57,17 @@ void NAME (void *opaque, struct st_sample *ibuf, struct st_sample *obuf,
         return;
     }
 
-    while (obuf < oend) {
+    /* without input samples, there's nothing to do */
+    if (ibuf >= iend) {
+        *osamp = 0;
+        return;
+    }
 
-        /* Safety catch to make sure we have input samples.  */
-        if (ibuf >= iend) {
-            break;
-        }
+    ilast = rate->ilast;
 
-        /* read as many input samples so that ipos > opos */
+    while (true) {
 
+        /* read as many input samples so that ipos > opos */
         while (rate->ipos <= (rate->opos >> 32)) {
             ilast = *ibuf++;
             rate->ipos++;
@@ -78,6 +78,11 @@ void NAME (void *opaque, struct st_sample *ibuf, struct st_sample *obuf,
             }
         }
 
+        /* make sure that the next output sample can be written */
+        if (obuf >= oend) {
+            break;
+        }
+
         icur = *ibuf;
 
         /* wrap ipos and opos around long before they overflow */