Consider changing `ssrCount` to something like `ssrViewportSize`

[inokawa]

Is your feature request related to a problem? Please describe.
Definition with px might be better than with count in some case, to handle dynamically sized items (e.g. #378 ).

Describe the solution you'd like
Create a new prop to define viewport size for SSR, and replace ssrCount with it. Probably it also requires a prop to define item sizes in SSR pass (#836 (comment)).

Describe alternatives you've considered
Continue to use ssrCount.

Additional context
N/A