Light paths overlay

[oktomus]

I rebased the code from @termhn and I will fix some issues before merging it. This PR brings the following:

• 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/

[dictoon]

Clean-ups:

• Only the beauty tab needs to be darken when rendering starts

Bugs:

• Crash when enabling light paths with a small render region
• Set render region button does not work
• File new crashes in LightPathRecorder::get_light_path_count
• New trigger assert in LightPathLayer
• Light path toolbar appear as a window when recreate_handlers is triggered
• Opengl error messages are showing up at runtime
• Make light paths thinner and keep thickness constant.
• Assert when clicking on "Next light path".
• Cannot select light paths in OpenGL visualization mode -> We don't need to support this use case for now.
• Move OpenGL visualization mode to separate tab (like the current "RGB" tab) and get rid of "Scene Display Mode" combo.
• What does Alt+LMB do? Can we disable it? -> Nothing
• Shift+LMB shouldn't select light paths.
• RMB shouldn't select light paths. -> The light paths picker is directly linked to the scene picker. It must not as these are 2 different things that should act differently. The light path picker must be independent and handle mouse inputs by itself.
• We need a way to clear light paths selection (Escape?).
• Clicking to select an area selects and shows light paths on the first click (instead of when the area has been defined).
• Save Light Paths button's tooltip should show the total number of available light paths, not the number of selected light paths.
• Backface culling in OpenGL view no longer works.
• Paths disappear when enabling backface culling
• Backface culling button should only be present in the OpenGL tab.
• If a render is aborted, are the old light paths properly discarded? It seems that we are still able to visualize the old light paths, in particular in tiles that weren't even rendered.
• Are we always 100% precise when clicking on a pixel?
• "Display Light Paths Overlay" gets disabled whenever a new render is triggered.
• Light paths-related messages are displayed in the log even when light paths recording is disabled.
• Clear Frame should also discard light paths.
• In OpenGL view, some light paths are shown through surfaces (see screenshot [1] below).
• In OpenGL view, when moving in front of the camera, light paths are no longer displayed correctly, they seem to go in the wrong direction (see screenshot [2] below).
• When visualizing a particular light path, other light paths should be either dimmed a lot more, or desaturated, or both: this requires experimentation (see screenshot [3] below).
• Regression: the Left and Right arrow keys should allow to visualize the previous/next light path, this no longer works.
• The "Display Transform" combo no longer updates the render widget.
• Toggling "Display Light Paths Overlay" off then on again should show again the light paths, right now they're lost whenever this is toggled.
• The Z-buffer from the OpenGL view is used, but if the OpenGL view is modified, then depth tests in the render no longer make sense (see screenshots [4] and [5] below).

Ignored tasks :

• A texture store is created every time a light path is selected? -> not related: -> Don't instantiate a TextureStore each time we pick something in the scene #2754
• What is the shading/lighting model of the OpenGL render? It used to be based on the facing ratio but now there appears to be some kind of lighting. -> not related: bug introduced in 2.1.0 -> Fix lighting regression in OpenGL mode #2755

Improvements for separate PRs:

• Allow the user to choose the number of light paths to show.
• Save light paths asynchronously to avoid blocking the UI.
• Use transparency to convey light paths intensity.
• Change "Display Light Paths Overlay" to icon.
• Can the light paths antialiasing be nicer?
• Zooming too much in the render (with the mouse wheel) leads to black screen

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[1]

[2]

[3]

[4]

[5]