fix crash when dealing with gif wallpapers

[MayonnaiseTraveler]

Problems:

• wpgtk crashes on launch after having selected a gif as a wallpaper.
• wpgtk does not update the wallpaper preview properly if it is a gif (happens if you launch it with a normal wallpaper selected and then try to select a gif wallpaper).

Program Output:

[i] theme_picker  current wallpaper: GiantSkeletonCleanSpriting2_small.gif
Traceback (most recent call last):
  File "/usr/bin/wpg", line 33, in <module>
    sys.exit(load_entry_point('wpgtk==6.5.5', 'console_scripts', 'wpg')())
  File "/usr/lib/python3.10/site-packages/wpgtk/__main__.py", line 309, in main
    _gui.run(args)
  File "/usr/lib/python3.10/site-packages/wpgtk/gui/theme_picker.py", line 196, in run
    win = mainWindow(args)
  File "/usr/lib/python3.10/site-packages/wpgtk/gui/theme_picker.py", line 82, in __init__
    self.pixbuf_preview = GdkPixbuf.Pixbuf.new_from_file_at_scale(
gi.repository.GLib.GError: gdk-pixbuf-error-quark: Nem todos os quadros da imagem GIF foram carregados. (6)

Part of the error is in Portuguese, the translation of "Nem todos os quadros da imagem GIF foram carregados. (6)" would be "Not all frames of the GIF image were loaded (6) "

Steps to reproduce :

• add gif image to wpgtk using thunar's search
• set theme with the gif as the wallpaper
• close wpgtk
• should crash next time you open it.

Fix suggestion:

Check if the file extension is '.gif' and get a static image from the gif for the preview.

[deviantfero]

is it possible you could refactor that logic into a method within that class?

[MayonnaiseTraveler]

sure, it will take a few days until i have time to do that, but once it's done i will update you!

[MayonnaiseTraveler]

@deviantfero

is it possible you could refactor that logic into a method within that class?

Done!

Separated the image preview loading code into two new methods: set_image_preview() is called when the combo box updates the selected wallpaper, and get_image_preview() is called when the program loads the previously selected wallpaper.

[deviantfero]

thanks for the contribution!