[WIP] Canvas rendertarget emulation

[scheibel]

This PR adds RenderTarget emulation for the Canvas for render targets a render stage requests, although the target framebuffer does not have them.
The list of features added includes:

• Emulate depth attachment if a target framebuffer has no such attachment
• Emulate stencil attachment if a target framebuffer has no such attachment
• Emulate combined depth-stencil attachment if a target framebuffer has no such attachment
• If a render stage requests one type of render target and provides an output of same type but different identity, apply a blit (future use case: feedback cycles, external pre-z pass control)

This PR depends on #368 (actual diff view).