Recursive scenes!

[dexterlb]

Allow including another stage as a layer in a scene.

This will make creating preview/multiview/etc scenes a breeze, in addition to fancy recursive scenes.

It should not be that hard to implement:

• (convenience) add a "null" sink type that just renders frames but doesn't send them anywhere
• Topologically sort the render targets so that they are rendered in the correct order
• Use the proper texture IDs for layers that refer to previously-rendered stages so that they get composited properly "for free" (without using the CPU)

[MartijnBraam]

This would be easily doable if it's done the same as on video mixer hardware. Every output is a "M/E" and every M/E can use the output of all the previous M/Es as a source.

This is not too hard to implement in the shader since the render targets are textures and with the PBOs you can use the non-locked buffers from the previous frames for rendering.