[fixed] No-extension filename failed to cooperation with renderer.

[tkbd]

In the state of unsaved Wings data, export file name is blank.
If a file name is blank,an user may save the file without the extension.
Especially such file makes fail to cooperation between a renderer and Wings3D.

An error recreate procedure(For example,use Yafaray exporter).

1. File>New.. make some 3D object (It does not save.)
   
2. File＞Render>Yafaray... Click OK button in setting dialog
   
3. Next,File Dialog is appear but filename box is blank.
   
4. so input "render_test" in filename text box, and do rendering.
   

Result:

• Rendering is success,then "render_test.tga" is saved.
  
• But Wings3D indicated an error message window 'No Image rendered'.
  
• Also, an item of "<<rendered>>" (Image data) doesn't add to outliner.
  

(It was tried with Wings3D 2.0.1 on OSX)

Suggestion:
I think that necessary to devise for the user does not make mistakes.
For example,when unsaved state, to autofill the filename "Untitled" and extensions in export dialog.
Untitled.tga
Untitled.bmp
Untitled.svg
etc...

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tkbd