[yt-dev] Issue #1129: [experimental] Opaque sources fade volume sources (yt_analysis/yt)

[Suoqing Ji]

New issue 1129: [experimental] Opaque sources fade volume sources
https://bitbucket.org/yt_analysis/yt/issues/1129/experimental-opaque-sources-fade-volume

Suoqing Ji:

When opaque sources are rendered with ``alpha=1`` in default, the volume sources would not be visible.

In plane parallel lens, the objects in the middle are not visible.

![test_plane-parallel.png](https://bitbucket.org/repo/BXbAb/images/573881956-test_plane-parallel.png)

In perspective lens, the entire volume sources are not visible.

![test_perspective.png](https://bitbucket.org/repo/BXbAb/images/3231665307-test_perspective.png)