hwcontext_vaapi: Do not assume that sw_format is transferable

Drivers can support a format for surfaces without also supporting it for
images, so we can't assume that sw_format is usable for transfer.  This
would previously hit an assert in cases where it isn't.

libavutil/hwcontext_vaapi.c

@@ -627,24 +627,31 @@ static int vaapi_transfer_get_formats(AVHWFramesContext *hwfc,
                                       enum AVPixelFormat **formats)
 {
     VAAPIDeviceContext *ctx = hwfc->device_ctx->internal->priv;
-    enum AVPixelFormat *pix_fmts, preferred_format;
-    int i, k;
+    enum AVPixelFormat *pix_fmts;
+    int i, k, sw_format_available;
 
-    preferred_format = hwfc->sw_format;
+    sw_format_available = 0;
+    for (i = 0; i < ctx->nb_formats; i++) {
+        if (ctx->formats[i].pix_fmt == hwfc->sw_format)
+            sw_format_available = 1;
+    }
 
     pix_fmts = av_malloc((ctx->nb_formats + 1) * sizeof(*pix_fmts));
     if (!pix_fmts)
         return AVERROR(ENOMEM);
 
-    pix_fmts[0] = preferred_format;
-    k = 1;
+    if (sw_format_available) {
+        pix_fmts[0] = hwfc->sw_format;
+        k = 1;
+    } else {
+        k = 0;
+    }
     for (i = 0; i < ctx->nb_formats; i++) {
-        if (ctx->formats[i].pix_fmt == preferred_format)
+        if (ctx->formats[i].pix_fmt == hwfc->sw_format)
             continue;
         av_assert0(k < ctx->nb_formats);
         pix_fmts[k++] = ctx->formats[i].pix_fmt;
     }
-    av_assert0(k == ctx->nb_formats);
     pix_fmts[k] = AV_PIX_FMT_NONE;
 
     *formats = pix_fmts;