Bug 1009306 - Allow invalidation when a layer's sub-pixel scroll changes. r=roc

We #ifdef'd this out before the implementation of flexible scroll ranges,
but now there's no need for this and it ends up causing visible rendering
glitches.
Bug 1009306 - Always align scrolling to pixels on mobile. r=roc

Whole-layer invalidations are always expensive, but they're especially
expensive on mobile. Avoid them on mobile by always aligning to pixel
values, even if it means the scroll position will fall out of bounds.