Update glow and remove it from the public API

[ryanisaacg]

Users shouldn't be subjected to breaking golem changes for every single glow breakage (which are pretty common, because glow has a bigger API surface.)

Additionally, stdweb is not longer a priority because it is unmaintained. Only web-sys will be supported going forward.

Closes #33

[coolreader18]

What if, instead of manually having the cfg for every instantiation, there was a function on blinds::Window like fn gl_initializer(&self) -> {impl FnMut(&str) -> *const c_void OR web_sys::WebGLRenderingContext} and a function on golem::Context like fn from_gl_init(init: {impl FnMut.. OR web_sys::...}). That way it's sort of just a convention (i.e. no semver dependencies), and it's still easy to create a golem context from a blinds window.