Merge pull request #1 from timjb/master

Code etwas verschönert, Pausieren implementiert
Merge pull request #2 from timjb/master

it's only possible to pause when a new tetromino appears
improve graphic performance by using a display list
proper pausing
holes are shown as black wireframe cubes
showing all relevant endges
display lists for all static cubes and all relevant edges
randomized tetromino spawning rotations