Previously, every RenderPass would have a bitfield that specified its type. That limits the number of passes to 32, which was reached a while ago.
However, most of the code already supported arbitrary RenderPasses since they were also used to store Multilayer EXR images.
Therefore, this commit completely removes the passflag from RenderPass and changes all code to use the unique pass name for identification.
Since Blender Internal relies on hardcoded passes and to preserve compatibility, 32 pass names are reserved for the old hardcoded passes.
To support these arbitrary passes, the Render Result compositor node now adds dynamic sockets. For compatibility, the old hardcoded sockets are always stored and just hidden when the corresponding pass isn't available.
To use these changes, the Render Engine API now includes a function that allows render engines to add arbitrary passes to the render result. To be able to add options for these passes, addons can now add their own properties to SceneRenderLayers.
To keep the compositor input node updated, render engine plugins have to implement a callback that registers all the passes that will be generated.
From a user perspective, nothing should change with this commit.
Differential Revision: https://developer.blender.org/D2443
Differential Revision: https://developer.blender.org/D2444
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