Ventcrawling fixes

Created by: ComicIronic

Ventcrawling now correctly starts from any pipe, not just vents and scrubbers. This has allowed me to fix moving between pipelines : the images are now correctly updated when you change pipes. Pipes now store their own images - this should help with processing cost, and lets them delete the image when they get deleted. You can now see pipes in darkness. Pipes are now readded when you log in as a ventcrawling mob. Added a small sanity to remove_ventcrawl(). Also edited another thing that Pomf never got around to. For a demo: http://puu.sh/hzTHa/d891c43328.webm

@RemieRichards