Small refactoring of graphviz_draw.

The only observable effect should be that calling with output=None and (output_format="xlib" or fork=True) no longer causes an AttributeError.
4 jobs for patch-3 in 113 minutes and 35 seconds (queued for 3 seconds)
Status Name Job ID Coverage
  Test
passed job_clang_amd64 #5033
amd64

00:59:30

passed job_clang_cxx_amd64 #5032
amd64

00:31:09

passed job_gcc_amd64 #5030
amd64

00:54:04

failed job_clang_amd64 #5031
amd64

01:05:00