This used to be a feature before the accurate-portable split, I'm
just restoring it.
Previously, while the EXE itself had a unique icon, the window and
taskbar both used the generic 'small' icon, which the original EXE
only used for the window.
SDL2 gives us a way to assign separate icons to each, but it's a
little clunky: it's Windows-only, requires the icons be in .ico
format, and needs them to be embedded in the EXE as resource files.
Also, for some reason, SDL2 doesn't let us refer to them by name - we
have to use their numerical ID.
976e2425fc