The icon artwork is a full-bleed dark tile, so every surface that shows it
without a mask of its own (the desktop install, the taskbar) drew a square.
Give the alpha channel a rounded-rectangle mask at 22.37% of the side — the
iOS/Android icon radius — sampled 4x so the corner reads as a curve rather
than a staircase.

The maskable pair is a byte-for-byte copy of the plain pair, so it is rounded
the same way; the transparent corners fall well inside any mask a launcher
applies. The apple-touch-icon is left alone: iOS masks it itself.