This is common when widget gets a zoom request, resulting in a
new tile buffer, and the tiles from the old tile buffer are still
waiting to be processed in the LOK thread, for old tile buffer. If
we allow these useless operations to execute successfully, they
would end up writing in new tile buffer giving false results.

Lets tag every paint tile operations with their respective tile
buffer during `task` creation, and then check whether the tile
buffer has changed or not before writing to the tile buffer.

Change-Id: If784341a67ad430bc3415b765137badaad6b97f6
Reviewed-on: https://gerrit.libreoffice.org/19726Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Miklos Vajna <vmiklos@collabora.co.uk>