Bug 1000266. Don't purge the canvas fixed background image cache if we are only scrolling. r=mattwoodrow

Also NotifyRenderingChanged was on the canvas background color item, not the background image item.

Bug 818643 was a problem where the cache didn't get purged enough, but the fix meant we basically always purged the cached and never got to use it, making it useless. It meant that we purged the cache anytime the item has any type of invalidation whatsoever, even if the item would be drawn the same but the layer contents needed to be invalidated so that it would be redrawn (ie due to staying at the same position on screen but a different position in the layer).

This is really hacky, but NotifyRenderingChanged is only observed on one type of display item. So we just isolate the case where the only thing that changed is the offset (due to scrolling) and skip the NotifyRenderingChanged in that case.