[css-ruby] Propose to make inter-character text container zero-sized

The current spec says:

When ruby-position indicates inter-character annotations, ... The affected
ruby annotation container is similarly sized so that its content box
coincides with that of the ruby base container.

This will cause a very undesired problem that the text container will block
any pointer event to reach the base texts, because it is always covered
over the base container.

Hence I propose that, the content box of inter-character text container
should be empty.

- Xidorn