wallpapers/potd: Add portrait orientation support for Bing provider
Detect portrait displays (height > width) and fetch 1080x1920 images for them. This provides better-composed images for vertical displays instead of center-cropped landscape images.
Resolution selection is done in the following order:
- Portrait (h > w): 1080x1920
- UHD (w > 1920 || h > 1080): 3840x2160
- Standard landscape: 1920x1080
Vertical UHD images are unfortunately not available.
Comparison screenshots (although today it's not a big difference)
Current (cropped from horizontal):
Vertical:
