CZML billboard and path with alpha-channel and enabled OIT in 2D

Hi, guys!

We are testing satellite CZML with semi-transparent white orbit path and facing the following issue: satellite's billboard doesn't overlay the path in 2D scene mode, and the path is blacked there (though everything is fine in 3D scene mode):


Turning off the OIT solves the issue for 2D.

I saw few OIT issues like #2441 and #2129 but have doubts whether my issue is linked with them. Is it hardware specific problem or not? It also persists on machines of a couple co-workers.

My setup:
- ASUS X73SM;
- Ubuntu 14.04 (64-bit);
- Google Chrome 44.0.2403.155 (64-bit);
- NVIDIA GeForge GT630M (NVIDIA proprietary drivers 340.76);
- Cesium 1.12.

CZML sample is available [here](https://drive.google.com/file/d/0B-KzCR6v7_RZbm1ZNEJlcURpRDg/view?usp=sharing).

Start by reproducing the linked CZML sample in Cesium 1.12 with OIT enabled, comparing 2D and 3D scene modes and then disabling OIT. Investigate the billboard and semi-transparent path rendering behavior; done means the billboard overlays the path correctly and the path is not black in 2D with OIT enabled.

Written by the indexing model from the issue text.