More flexible auto-rotation

[kpreid]

The system for rotating blocks when they are placed should be made more flexible. The current system chooses the best of a set of allowed orientations, according to the alignment of the -Z direction with the player's view direction; this should be changed to permit such things as

• alignment with an axis other than the Z axis (so that the orientation in the block menu is independent)
• defining rotation based on the player's position relative to the new block, rather than the aim direction

The rotation rules should be defined by by circuits if that turns out to be feasible. (Vector operations in circuits?)

Another possibility would be to have a scheme for directly choosing the orientation before or after placement.