gst_debug_bin_to_dot_file() NOOPs if GStreamer was initialized without
the enviroment variable GST_DEBUG_DUMP_DOT_DIR.
If SIGUSR2 was signalled we want to have a dot file dumped no matter
what. Functions for asynchronously writing the graph as obtained by
gst_debug_bin_dot_data() to a temporary file and print the path to
stdout.
Note: Usage of GST_DEBUG_BIN_DOT_FILE() on pipeline state change is left
as is, because we don't want to dump graphs for every change on every
pipeline.
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