fix not clearing when given invalid values, from #1756

[gwyneplaine]

Cleaning up and adding tests to @ticklemynausea's initial PR #1756

[coveralls]

Coverage increased (+0.02%) to 92.89% when pulling 8ebda3a on fix/renderClear into ca09183 on master.

[JedWatson]

Thanks

@gwyneplaine it looks like this can be optimised so we don't need to call getValueArray in every render - can you look into a subsequent pr that fixes this?