Specify fallback model for performance issues

[marcoscaceres]

We should make sure the spec says something about the browser being able to drop the model on the floor if it's consuming too many resources or will make the user uncomfortable.

[DR-xR]

The browser should really fail gracefully. Dropping it on the floor, especially without warning or notice, can be bad too.

[cabanier]

During the meeting, people mentioned that browser currently don't specify what happens when images are too big or if there are too many so we don't need to do anything. However, the model tag is different because a lot of existing content will not be renderable at a acceptable frame rate by existing and near future headsets. We should require that there is a image fallback in the element in case the model is too big or not renderable or if there are too many.