Rendertarget interfaces

[scheibel]

ToDo:

• RenderTarget and RenderInterface hardening (make resistant to unconnected slots and nullptr-RenderTargets)
• Complete documentation on changed/introduced interfaces
• Redesign RenderTarget and RenderInterface interfaces
• Add blit to canvas if pipeline doesn't render in provided color RenderTarget
• Check all stages that renders using the RenderInterface
• Check all usages of RenderInterface if the more-specific CanvasInterface may be used
• Move input/output queries to Stage interface
• Fix BlitStage logic
• Introduce subclasses for RenderTarget
• Document workarounds in pipeline design using RenderTarget interface
• Framebuffer management by RenderInterface
• Add DepthStencilRenderTarget subtype to handle combined depth-stencil attachments correctly.

Delayed tasks:

• Refactor ClearStage to handle more data types in a more idiomatic order (handled in Clearstage refactoring #379)
• Emulate framebuffer attachments if target framebuffer cannot satisfy render stage requirements (handled in [WIP] Canvas rendertarget emulation #380)

[scheibel]

Current state: ShapeDemo is working using a RenderTarget instead of Framebuffer interface.

[scheibel]

I think the first milestone for rendertarget interfaces is reached and this PR is now available for review and merging.
There are some open tasks which I want to address later as separate issues.

[scheibel]

I merged the current master. This PR is now reviewable/mergable.

I fixed most of the old example pipelines/stages, too.
However, I suggest @Tobias1595 has a further look on the examples that should remain.