fixes node timer bug (fixes #407).

Previously, when a block was activated, on_timer callbacks where
called with the relative position of the node inside the block,
instead of the absolute position of the node.
fix integer overflow (fixes #414)

liquid_kind was declared as an u8, but used to hold a content_t value, which is delcared to be a u16.

changing this fixes (at least for me) the problem reported in bug #414.
Fix spread of items in falling code