Fix crash when using list layout (#68).

- Cause and fix found by @mrjjwright.
Make it possible to clear a cell’s background.

- Before this change it was not possible to clear a cell’s background
after having set either the background image or color.
Merge pull request #71 from rastersize/fix-list-layout-calloc

Fix crash when using list layout (#68).
Merge pull request #72 from rastersize/clear-cell-background

Make it possible to clear a cell’s background.
Merge remote-tracking branch 'upstream/master'