Implement extendable view transforms

[ammen99]

For example, a 3D transform can be used in conjunction with a 2D one (so, switcher should be able to use the window rotation given by e.g wrot). Also, once #20 is done, wobbly might want to extend the 2D transform as well.

There are two ways to do this - render the view with the first transform to an FBO, then render the FBO to a second FBO with the second transform, and so on. This means we could compose any number of transforms, but might mean that they might not really integrate well. And, if we had window exploding (a type of closing animation) its dubious whether we actually want to enable yet another transform.

The other way: 2D(and possibly 3D) transforms are the "base", plugins can request that they are "kept" active (like ref counting), and the transforms can be dynamically converted into a "derived" custom transform (for example wobbly or explosion transform) and then back to a plain 2D or 3D.

[ammen99]

implemented in wlroots branch