fix: properly access multipass filename from render data

Resolves #38

What was the problem/requirement? (What/Why)

When opening the submitter in a scene that saves multipass output, the submitter would always crash regardless if there any info or not.

Deadline UI launch failed
Traceback (most recent call last):
  File "C:\Program Files\Python310\Lib\site-packages\deadline\cinema4d_submitter\cinema4d_render_submitter.py", line 54, in show_submitter
    w = _show_submitter(None)
        ^^^^^^^^^^^^^^^^^^^^^
  File "C:\Program Files\Python310\Lib\site-packages\deadline\cinema4d_submitter\cinema4d_render_submitter.py", line 273, in _show_submitter
    output_directories = Scene.get_output_directories(take=take)
                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Program Files\Python310\Lib\site-packages\deadline\cinema4d_submitter\scene.py", line 152, in get_output_directories
    path = render_data.GetFilename(c4d.RDATA_MULTIPASS_FILENAME)
           ^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'c4d.documents.RenderData' object has no attribute 'GetFilename'

What was the solution? (How)

We were accessing it as if the render data was the data container, rather than the data instance. Swapped to the data instance method

What is the impact of this change?

Submitter now properly loads when the scene has multipass data

How was this change tested?

Reproduced issue locally
made the change and installed
verified the submitter no longer crashes when loading

Was this change documented?

N/A

Is this a breaking change?

N/A

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aws-deadline / deadline-cloud-for-cinema-4d

fix: properly access multipass filename from render data #47