Fixes https://github.com/flutter/flutter/issues/142183

This fixes a bug in the SliverGeometry of SliverMainAxisGroup. The cacheExtent represents how many pixels the sliver has consumed in the SliverConstraints.remainingCacheExtent. Since it was not set, slivers that came after a SliverMainAxisGroup that filled the whole screen did not properly lay out their own children, in some cases making lazy sliver more eager than they should be.