ffmpeg: Make filter hardware device selection clearer

Also move it into a dedicated function in the hardware file.

fftools/ffmpeg.h

@@ -661,6 +661,7 @@ void hw_device_free_all(void);
 
 int hw_device_setup_for_decode(InputStream *ist);
 int hw_device_setup_for_encode(OutputStream *ost);
+int hw_device_setup_for_filter(FilterGraph *fg);
 
 int hwaccel_decode_init(AVCodecContext *avctx);
 

fftools/ffmpeg_filter.c

@@ -1061,17 +1061,9 @@ int configure_filtergraph(FilterGraph *fg)
     if ((ret = avfilter_graph_parse2(fg->graph, graph_desc, &inputs, &outputs)) < 0)
         goto fail;
 
-    if (filter_hw_device || hw_device_ctx) {
-        AVBufferRef *device = filter_hw_device ? filter_hw_device->device_ref
-                                               : hw_device_ctx;
-        for (i = 0; i < fg->graph->nb_filters; i++) {
-            fg->graph->filters[i]->hw_device_ctx = av_buffer_ref(device);
-            if (!fg->graph->filters[i]->hw_device_ctx) {
-                ret = AVERROR(ENOMEM);
-                goto fail;
-            }
-        }
-    }
+    ret = hw_device_setup_for_filter(fg);
+    if (ret < 0)
+        goto fail;
 
     if (simple && (!inputs || inputs->next || !outputs || outputs->next)) {
         const char *num_inputs;

fftools/ffmpeg_hw.c

@@ -480,3 +480,31 @@ int hwaccel_decode_init(AVCodecContext *avctx)
 
     return 0;
 }
+
+int hw_device_setup_for_filter(FilterGraph *fg)
+{
+    HWDevice *dev;
+    int i;
+
+    // If the user has supplied exactly one hardware device then just
+    // give it straight to every filter for convenience.  If more than
+    // one device is available then the user needs to pick one explcitly
+    // with the filter_hw_device option.
+    if (filter_hw_device)
+        dev = filter_hw_device;
+    else if (nb_hw_devices == 1)
+        dev = hw_devices[0];
+    else
+        dev = NULL;
+
+    if (dev) {
+        for (i = 0; i < fg->graph->nb_filters; i++) {
+            fg->graph->filters[i]->hw_device_ctx =
+                av_buffer_ref(dev->device_ref);
+            if (!fg->graph->filters[i]->hw_device_ctx)
+                return AVERROR(ENOMEM);
+        }
+    }
+
+    return 0;
+}