Beginnings of Servo -> MediaDecoder interface

[cpearce]

This isn't quite complete, but I'm requesting merge, as otherwise everyone else will also need to rewrite this code in order to get anything done.

• Adds an id to each instance of GeckoMedia. We'll use this id to uniquely identify MediaDecoder instances on the C++ side of the FFI boundary.
• In future when a GeckoMedia instance needs to call a function on a MediaDecoder instance, it must pass the id over the FFI boundary, and the C++ code can lookup the instance using the id in a hash table or somesuch, and call the corresponding function (this code isn't there yet).
• Adds MediaDecoder_Load() and MediaDecoder_Unload() C functions which internal Rust code can call. These assume we're passed a Vec to play, as what Servo can pass for BlobURIs today.
• Switch to passing a table of functions over the FFI boundary rather than declaring every Rust function we want C++ to be able to call as "pub".

[nox]

Adds an id to each instance of GeckoMedia. We'll use this id to uniquely identify MediaDecoder instances on the C++ side of the FFI boundary.

To each instance of GeckoMedia? But I thought GeckoMedia was the entry point for things that are not related to any decoder? I'm really confused now.

[bors-servo]

:umbrella: The latest upstream changes (presumably #52) made this pull request unmergeable. Please resolve the merge conflicts.

[cpearce]

This is obsoleted by PR52.