Bug 1662646 - Remove `RendererOptions::batch_lookback_count`. r=gw
Bug 1662646 - Remove `RendererOptions::batch_lookback_count`. r=gw

It's never set to anything other than the default, so we can hardwire it as a
constant.

Depends on D89100

Differential Revision: https://phabricator.services.mozilla.com/D89101

[ghsync] From https://hg.mozilla.org/mozilla-central/rev/1a01aebd9f63f34474d6850e2eab024e7304706b

servo/webrender,servo/webrender,servo/webrender,servo/webrender,servo/webrender,servo/webrender