hw_base_encode: allocate DPB image upfront

Vulkan requires this, as it needs to initialize state upfront.

libavcodec/d3d12va_encode.c

@@ -264,12 +264,6 @@ static int d3d12va_encode_issue(AVCodecContext *avctx,
 
     av_log(avctx, AV_LOG_DEBUG, "Input surface is %p.\n", pic->input_surface->texture);
 
-    err = av_hwframe_get_buffer(base_ctx->recon_frames_ref, base_pic->recon_image, 0);
-    if (err < 0) {
-        err = AVERROR(ENOMEM);
-        goto fail;
-    }
-
     pic->recon_surface = (AVD3D12VAFrame *)base_pic->recon_image->data[0];
     av_log(avctx, AV_LOG_DEBUG, "Recon surface is %p.\n",
            pic->recon_surface->texture);

libavcodec/hw_base_encode.c

@@ -446,6 +446,12 @@ static int hw_base_encode_send_frame(AVCodecContext *avctx, FFHWBaseEncodeContex
             goto fail;
         }
 
+        err = av_hwframe_get_buffer(ctx->recon_frames_ref, pic->recon_image, 0);
+        if (err < 0) {
+            err = AVERROR(ENOMEM);
+            goto fail;
+        }
+
         pic->priv = av_mallocz(ctx->op->priv_size);
         if (!pic->priv) {
             err = AVERROR(ENOMEM);

libavcodec/vaapi_encode.c

@@ -313,11 +313,6 @@ static int vaapi_encode_issue(AVCodecContext *avctx,
 
     av_log(avctx, AV_LOG_DEBUG, "Input surface is %#x.\n", pic->input_surface);
 
-    err = av_hwframe_get_buffer(base_ctx->recon_frames_ref, base_pic->recon_image, 0);
-    if (err < 0) {
-        err = AVERROR(ENOMEM);
-        goto fail;
-    }
     pic->recon_surface = (VASurfaceID)(uintptr_t)base_pic->recon_image->data[3];
     av_log(avctx, AV_LOG_DEBUG, "Recon surface is %#x.\n", pic->recon_surface);