WinRT bitmap cleanup

This PR is a follow on to #19, to clean up a few COM-related issues.

@rzhw, I've added you as an owner to Splat - can you check out this branch and make sure I haven't broken anything?  I don't really use WinRT so I don't have a good test app to try (and I never repro'ed the bug). I'm pretty sure this is fine though. 

Since you're now an owner, if you find anything broken, you should be able to just commit directly to this branch.

Fixes #19 when this is merged.