New crop

[bremner]

Here's a first step to making the overlay / crop action more flexible. It reads both the video size and slide image size from the corresponding files. There are still assumptions here about about how to crop and scale the video (to a central square, then scale by 0.5) and where to put it (bottom right). Presumably more attributes for the video tag are needed to control this.

I guess this could probably close #1, although it could be left open to track future enhancements.

The tests I added are mostly pretty simple minded. I don't have a good idea how to easily verify the output of melt is sane, but the tests do give things to run by hand also.

this PR includes PR #3.