This helps ensure that images are swapped in when we actually
render tiles. Previously we'd sometimes have placeholders
instead of the image, which results in either an invalidate
(+rerender of that tile) once the image is swapped in (for
normal tiles) or a permanently missing image in the preview tiles.

Change-Id: I1a16a913faf9fad20e40a5d1aad3de187038c7a2
Reviewed-on: https://gerrit.libreoffice.org/19890Reviewed-by: Andrzej Hunt <andrzej@ahunt.org>
Tested-by: Andrzej Hunt <andrzej@ahunt.org>