Added function to enable dragging.
Merge branch 'master' into changeHeight-by-dragging
Completed changeHeight by dragging.

It is not 1-on-1 with the input coordinates, due to restrictions in the
communication protocol (we cannot send how much the terrain height should
change). Instead, dragging upwards causes the terrain to rise, and dragging
downwards causes the terrain to drop (this is determined from the initial starting
position).
Merge branch 'changeHeight-by-dragging'