gallium: add pipe_picture_desc::flush_flags

It specifies the flush flags for pipe_video_codec::end_frame.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/28771>

src/gallium/include/pipe/p_video_state.h

@@ -191,6 +191,8 @@ struct pipe_picture_desc
    enum pipe_format input_format;
    bool input_full_range;
    enum pipe_format output_format;
+   /* Flush flags for pipe_video_codec::end_frame */
+   unsigned flush_flags;
    /* A fence used on PIPE_VIDEO_ENTRYPOINT_DECODE/PROCESSING to signal job completion */
    struct pipe_fence_handle **fence;
 };