Too much code duplication

[sagudev]

There is a lot of code duplication in surfman egl that could be avoided if we used traits and default implementations.

[mrobinson]

I think this crate could be a lot simpler if instead of having only a single implementation of every Surface and Device, each Surface and Device was internally an enum of the different backends -- the same way that raw_window_handle, etc are designed. That would also allow using more code reuse, I think.