Multiline selection doesn't get rendered property 

Hi Marijnh,

From time to time we encounter a problem with selection in codemirror which doesn't get rendered whenever it spans multiple lines. We don't have a stable repro, but debugging of the broken state showed that this is due to `display.chachedPaddingH` storing `NaN`s as values for both `left` and `right`.

The patch, unfortunately, doesn't fix the root cause of the problem, however it will make codemirror to self-recover from the bad state. If the root cause remains undetected, it will be nice to have this in.
What would you say?