Add replay API support

[ebkalderon]

It would be nice to support the RenderDoc replay API, which allows third-party RenderDoc compatible GUIs to be written. This API is written in C++ and is considered unstable by its designers, but the possibilities of exposing it to the Rust gamedev scene are tremendous, e.g. the Amethyst toolchain, among other projects.

[ebkalderon]

Un-assigning myself from this task for now, as it has lower priority at the moment.

[anholt]

I'm interested for removing the external renderdoc-using python script dep in my project. Do you have an outline of how to get started here? I've never played with bindgen, which I assume is part of the solution.

[anholt]

first evening of playing around with bindgen: it wants to process STL headers and choking --no-recursive-whitelist isn't helping. Also dropping the --blacklist-type isn't helping.

Also, we have to #define an appropriate RENDERDOC_PLATFORM_* so that will be a bit of fun to handle.