Unified Viewport

[fu5ha]

• Fixes some ways to get into invalid states by going into and out of light path display toggle/base layer modes and restarting rendering
• No longer does a light path picking event when moving the camera in OpenGL view
• No longer does a light path picking event at all when in OpenGL view until I implement better filtering modes/scene space picking
• Dims the final render when you have at least one light path selected

To be implemented in this PR:

• Selected individual light paths are not drawn solo but rather the rest of the picked light paths have lower transparency.

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https://www.grayolson.me/blog/posts/gsoc-2019/

[oktomus]

Closed in favor of #2748