lavc: Add per-thread surfaces in get_hw_frame_parameters()

This number is definitely required when frame threading is enabled, so
add it here rather than forcing all users to handle it themselves.

DXVA2 contained this addition in specific code as well (therefore being
added twice in the internal case) - just remove it from there.

libavcodec/decode.c

@@ -748,10 +748,6 @@ int ff_decode_get_hw_frames_ctx(AVCodecContext *avctx,
         // We guarantee 4 base work surfaces. The function above guarantees 1
         // (the absolute minimum), so add the missing count.
         frames_ctx->initial_pool_size += 3;
-
-        // Add an additional surface per thread is frame threading is enabled.
-        if (avctx->active_thread_type & FF_THREAD_FRAME)
-            frames_ctx->initial_pool_size += avctx->thread_count;
     }
 
     ret = av_hwframe_ctx_init(avctx->hw_frames_ctx);
@@ -798,6 +794,11 @@ int avcodec_get_hw_frames_parameters(AVCodecContext *avctx,
             // available then add them here.
             if (avctx->extra_hw_frames > 0)
                 frames_ctx->initial_pool_size += avctx->extra_hw_frames;
+
+            // If frame threading is enabled then an extra surface per thread
+            // is also required.
+            if (avctx->active_thread_type & FF_THREAD_FRAME)
+                frames_ctx->initial_pool_size += avctx->thread_count;
         }
 
         *out_frames_ref = frames_ref;

libavcodec/dxva2.c

@@ -608,10 +608,6 @@ int ff_dxva2_common_frame_params(AVCodecContext *avctx,
     else
         num_surfaces += 2;
 
-    /* add extra surfaces for frame threading */
-    if (avctx->active_thread_type & FF_THREAD_FRAME)
-        num_surfaces += avctx->thread_count;
-
     frames_ctx->sw_format = avctx->sw_pix_fmt == AV_PIX_FMT_YUV420P10 ?
                             AV_PIX_FMT_P010 : AV_PIX_FMT_NV12;
     frames_ctx->width = FFALIGN(avctx->coded_width, surface_alignment);