Logic issue in InsetTableCell::draw

[Jean-Marc Lasgouttes]

Dear all,

This one is for Jürgen. Coverity states it very clearly: "Local variable 
change_drawn is assigned only once, to a constant value, making it 
effectively constant throughout its scope. If this is not the intent, 
examine the logic to see if there is a missing assignment that would 
make change_drawn not remain constant. Otherwise, declaring change_drawn 
as const will suppress this defect."

I do not know enough of the intent to know how to fix it.

JMarc