Fixes#909
Since we declare the struct in C++, we don't need to declare it in
rust for cbindgen anymore, as long as we expose the Size2D type from
euclid to the cbindgen_private namespace
Add the graphics state enum to the callback, too. When we add support for different backends,
it would be nice if it didn't require an API change.
It's duplicated from Rust
because it doesn't provide values. The WebLG one doesn't make sense for C++ and
the proc address closure isn't ffi safe.
(It could be manually bridged thought)
This API allows specifying a callback that will be invoked when setting
up graphics (great for compiling shaders), before rendering a frame (but
after the clearning of the surface background), after rendering a frame
(before swapbuffers) and when releasing graphics resources.
