The bug seems to be in Windows when it upscales an smaller icon for
HiDPI displays. It works fine if we provide a 256x256 size in the
ico files.
Use the new square icons also used on OS X that removes the share badge
and instead has a share icon only for the OK state. This means that we
can also remove 3 icons and release 3 slots in the registry for
overlay icons (only the first few are actually loaded by explorer).
This also adds a script that uses ImageMagick to convert our SVG icons
to Windows ico files. Since ImageMagick seems to have issue doing
proper antialiasing with pixels on the edge of icons, this also
slightly scale the icons to leave 2px on each edge, out of its
logical 128px width.
