Sort out resolution, overscan, and pixel aspect ratio

[johnhaddon]

As discussed in ImageEngine/cortex#330.

[johnhaddon]

Is this a good time to add overscan support as well?

[andrewkaufman]

Do you have an idea for how overscan will be handled?

[johnhaddon]

Well, I don't think the IECore::Renderer subclasses should have to worry about it, so it wouldn't be a renderer option. I was thinking of a render:overscan option set by the StandardOptions node, and then in outputCamera() in RendererAlgo.h we'd adjust the resolution and screen window appropriately just before passing to the renderer. I don't have a strong opinion on how overscan should be specified - should it be % values or pixels? And should it be specified individually for each edge or just once overall? I guess % vs pixels is partly down to how we want to handle specifying half-res renders and the like - do we want to just change render:resolution explicitly, or would we prefer a render:resolutionMultiplier option? The latter seems to imply that we might want to use % rather pixels for overscan.

We haven't done anything about film fitting yet either - what to do if the aspect ratio from the resolution and from the camera frustum don't match. Now's probably a good time to do that too.

[johnhaddon]

Quick note that we'll also want a resolution multiplier as part of this.