Order Independent Transparency

Implementation of [Weighted Blended Order-Independent Transparency](http://jcgt.org/published/0002/02/09/).

This PR is for early feedback. There are a few issues that still need to be fixed.

There are two rendering paths: multiple render targets and multiple render passes. The multiple passes path will be used when multiple render targets are not supported. If you want to test the multiple render target path, enable the D3D11 back end and draft extensions in Chrome and check that the [WEBGL_draw_buffers](http://www.khronos.org/registry/webgl/extensions/WEBGL_draw_buffers/) extension is available at [WebGL Report](http://webglreport.com/).

Green extent primitive intersecting a red sphere:
![image](https://f.cloud.github.com/assets/1494815/2015363/725ea76c-87b8-11e3-93dc-bc134b52e88e.png)

Red sphere inside a green sphere:
![image](https://f.cloud.github.com/assets/1494815/2015372/8307a29e-87b8-11e3-8aa2-e3ebf7d02b6f.png)

Different color extruded extents, alpha decreases top-to-bottom/left-to-rigt:
![image](https://f.cloud.github.com/assets/1494815/2015378/c7a8897c-87b8-11e3-8d54-88d8765f078c.png)