generate_wallpaper_sizes.py: Properly compress wallpapers
The current options are quite bad and result in files which are unnessarily big, even bigger than the original file. The Pillow documentation says: > Values above 95 should be avoided; 100 disables portions of the JPEG > compression algorithm, and results in large files with hardly any gain > in image quality. "100" is not a valid value for the subsamping parameter either. This change actually enables proper compression with subsampling and also enables optimization of internal settings. Result: From 34MiB down to 12MiB with no noticable difference.