InputCamera: remove bug-prone constructor, ask explicitely for camera size.

This is a potentially code-breaking change, so I'm using the merge request mechanism to notify people in the team.

We had an InputCamera(Camera & cam) constructor which was leaving uninitialized the width and height of the new InputCamera, leading to hard-to-find bugs with random camera sizes. This was because Camera doesn't have the notion of image size, only the notion of aspect ratio.

The fix I decided upon is to modify the constructor to require the developer to specify a size explicitely. I also had to remove an assignment operator that could create the same issue. This means that you might encounter compilation errors when you were creating an InputCamera directly from a Camera. To fix those cases, you now have to pass a width and height to the constructor (something that you would have had to set on the next line with InputCamera::size(w,h) in the previous setup, if you were aware of it).

If everybody is aware of this breaking change and is fine with it, this could be merged in master.