Add a variant of StdwebGenerator with the same interface as StructGenerator

[nstoddard]

I'm trying to convert a project from wasm32-unknown-emscripten to wasm32-unknown-unknown, but in order to share the same rendering code on both desktop and wasm, I need OpenGL bindings with the same interface as StructGenerator. Previously, with emscripten, I could use StaticStructGenerator, but with wasm32-unknown-unknown, the only option seems to be StdwebGenerator, which, among other differences, generates function names like bind_texture instead of BindTexture. Would it be possible to add a variant of StdwebGenerator that doesn't have these differences?

[nstoddard]

I discovered a library called glow which does pretty much what I was looking for. It also turns out that there's many differences in function signatures between StructGenerator and StdwebGenerator, so it would probably be difficult to unify them from within gl-rs. Also, this is pretty much a duplicate of #476.