RenderTarget.canvas nested rendering

[rayzchen]

If RenderTarget.canvas is true, it may try to render itself in PreRender. Whether this is wanted or not, there needs to be a limit to the depth of recursion.

[rayzchen]

One suggestion is instead of rendering all Canvases per camera, assign a canvas to a single camera and then render only that canvas. Nesting canvases also allows for some basic merging of canvas rendering (for example, camera A renders canvases A and B while camera B only renders B). More complex set operations could be handled in other ways.

However a problem arises when the RenderTarget's source is set to a camera which renders the canvas it is inside, which still has this issue.

[rayzchen]

Merged