This makes for much faster lookups, and avoids the need
to allocate an array.
sizeof(BWindow) is unchanged (on x86_64, 504; on x86, 376)
so this should not break ABI/API.
Quick benchmark in Tracker:
501 lookups, before: ~488 usec
501 lookups, after: ~284 usec
Change-Id: Ica2c728f0c5957380add53e14a826a4aade52df0
Reviewed-on: https://review.haiku-os.org/c/haiku/+/10720
Tested-by: Commit checker robot <[email protected]>
Reviewed-by: waddlesplash <[email protected]>