Fix placement and autoconversion of slabs so the textures are rotated
correctly.  Fixed autorotation of panels onto walls also, made them and
microblocks respond to shift when placing on ground/ceiling to force
wall rotation.
Merge pull request #19 from VanessaE/master

Rotation of textures is wrong in placement and autoconversion of vertical slabs
Merge remote-tracking branch 'upstream/master'