Suppress redisplay on invalidation

Suppress redisplay on invalidation

We can end up in a situation when throttling is active, redisplay is
scheduled (if post-processing fails) and then invalidate is called -
resulting in an empty layout and consequently flickering.

This is probably the third time when post-processing intricacies
lead to nasty visual issues.

Reproducer:

mkfifo /tmp/fifo
llpp some.pdf -remote /tmp/fifo &
echo 'keys ctrl-0' >/tmp/fifo
for i in $(seq 1 100); do
    echo 'keys ctrl-+' >/tmp/fifo;
    sleep 0.01;
done;