Lets separate the task of painting the tile, and saving the tile
in tile buffer using GAsyncReadyCallback. This will provide us
with better control over tiles -- cancelling the painting operation,
and filtering tiles that should not be saved in the tile buffer.

Change-Id: I6aae928d8cc0c906034570ed0e9a054763d493a3
Reviewed-on: https://gerrit.libreoffice.org/19725Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Miklos Vajna <vmiklos@collabora.co.uk>