Use glow instead of gl_generator for GL/GLES bindings

[sagudev]

See servo/servo#33539.

Note that this PR will expose glow types in surfmans public API.

[sagudev]

So the problem is that:

thread_local! {
    pub static GL_FUNCTIONS: Gl = unsafe {Gl::from_loader_function(context::get_proc_address)};
}

does not work because glow requires for context to be current, because it does some querying (version, extension). My idea is to simply replace all usages of GL_FUNCTIONS with let gl = unsafe {Gl::from_loader_function(context::get_proc_address)}; as most of the time we are already current in the calls. This would make querying every time, but functions are not hot AFAIK, so this should be ok. Alternative would be to have Gl stored on context, but this needs to be lazy value, as context creation does not make context current.

[sagudev]

I am wondering do we really need newly-created contexts that not immediately made current, or can we just made them current on creation? Linked issue on test is about something different. THis was introduced in dae72e4. cc @pcwalton @jdm