Scaffolding tweaks

Scaffolding is an incredibly useful block. However, there are some tweaks to placement that would make it easier to deal with.

• If both your head and feet are in scaffolding, currently if you aim at the bottom scaffolding, the top will be aimed at. This should be fixed so that new scaffolding is placed from the one at your feet, so that you can easily branch of large pillars,
• If sneaking and jumping on scaffolding, it should act like sneaking on a solid block. Sneak-jumping is not that useful, whereas this would make it easy to place blocks next to scaffolding (without having to go down, extend a branch 1 block below, then back up).