[WIP] Basic slope support

I like slopes, which has made 3D mode a little less than useful, so I took a crack at this.

It even correctly avoids warning about missing textures in basin-style architecture!

Only understands `Plane_Align` and tilt things at the moment, because those are all I'm using at the moment.  Also has a lot of TODOs.  And could probably stand to compute the floor/ceiling planes _once_ rather than over and over all the time.  And some other things.  But it's a start.

I went off the beaten path a bit and wrote the slope-finding function as a `template<PlaneType>`, where `PlaneType` is just an enum of floor and ceiling.  A couple similarly-templated helpers took care of the details, and I only had to write the code once.  I'm hoping this can cut down on the need to copy/paste to handle both floors and ceilings.

![z8lfksf](https://cloud.githubusercontent.com/assets/94112/5057184/de9cb860-6c64-11e4-8c43-aba2d42c9acf.png)