render-lines: avoid looping indefinitely when shifting.

render-lines: avoid looping indefinitely when shifting.

If <rtab> is used for markup when wrap is enabled, and if the cursor is
at the end of the list, it is impossible to shift enough that there is
room for the cursor after the text.
So the shift loop continues endlessly.

Put a hard limit on this, and use a consistent limit in the other places
that we shift.

Signed-off-by: NeilBrown <neil@brown.name>