[custom-paint] First iteration of level 1 spec.

Hi all,

We've written a first (rough) version of the custom paint API spec. Greg,
Rossen, Shane, Tab & I sat down over VC today and did a read-through of any
issues initially that might arise.

Please have a read and let us know of any feedback on the API design and
potential issues.

http://dev.w3.org/houdini/css-paint-api/

Thanks,
Ian