[css-scroll-snap] snap alignment container (concept name)

There's an issue marked in the spec for a better name for this
concept:

https://drafts.csswg.org/css-snappoints/#definitions
   # snap alignment container
   #    A scroll container’s snap alignment container is the
   #    rectangle obtained by reducing its visual viewport
   #    by its scroll-snap-padding.
   #
   # ISSUE: Better name for this concept?

I propose “scroll snap window”, “snap window“ for short!
(I.e. use “snap window” but allow other specs to cross-link in to
“scroll snap window” for better context.)

Advantages:
   * shorter and easy to pronounce
   * relates to the concept of having a hole inside which we're
     viewing stuff, which is what it is
   * doesn't use the overloaded word “viewport”

Disadvantages:
   * ???

~fantasai