Improve `--bigsurify`

[fwcd]

Improve the --bigsurify flag (which automatically resizes/masks the icon to a rounded-rectangle shape):

• Generate smooth corners (perhaps though an antialias flag?)
• Only apply it to the --macos icon sizes (and generate e.g. iOS icons as-is)
  • This might require some more bookkeeping, e.g. we cannot share icons of same size between macOS and iOS, since macOS would require the rounded icon whereas iOS wouldn't
  • Implemented in Apply --bigsurify to macOS icons only #7
• Add a white (configurable?) background (for transparent, bigsurified icons?
• Add a slight drop shadow (also configurable)