Add new graphics to the bundled terrain sheets, and possibly reorganize the sheets

[CelticMinstrel]

I have about 7 different drafts of this work (terrain only) on my hard drive that were never checked in for reasons I can't even remember. There are a lot of different sources for the new graphics:

• Exile 1-3 – we have permission from Jeff Vogel and Andrew Hunter to use graphics from these games that didn't make it into BoE.
• Mistb0rn – ages ago he made a lot of custom graphics at my request, many based on Blades of Avernum graphics.
• Almighty Doer of Stuff – he made a ton of new custom graphics with the intent of including them in the game. Some of them have already been added, but I don't think all of them have.
• Pixel Profusion – there are a great many good graphics made by various people in the old Blades community. Some of them explicitly gave us permission to use their graphics, but I don't remember exactly who gave permission. It's probably recorded somewhere though. I couldn't find any record of it, but I'm pretty sure at least Milu agreed to let us use graphics. The only other people who made graphics of interest were ShyGuy and Tim Farland.

This should really be done before an official release is made, as changing the layout of the graphics sheets later will break any scenarios created in the new release.

If we're doing #413 it should probably be done before this, but it doesn't have to be – the transitions could be shunted to the end of the list instead.

This is the most recent version, but by no means is it final, nor anywhere close. It's also not settled that it'll be combined into one giant sheet like this. Some other revisions are available in the same directory.