Update to glutin v0.21.0

[iceiix]

Glutin v0.21.0 migration guide: https://gentz.rocks/posts/glutin-v0-21-0-migration-guide/

[iceiix]

Currently using a minor fork of glutin, iceiix/glutin#1
Updating to 0.21.0, errors:

error[E0432]: unresolved import `glutin::ContextTrait`
  --> src/gl/mod.rs:16:5
   |
16 | use glutin::ContextTrait;
   |     ^^^^^^^^^^^^^^^^^^^^ no `ContextTrait` in the root. Did you mean to use `ContextError`?

error[E0432]: unresolved import `glutin::ContextTrait`
  --> src/main.rs:55:5
   |
55 | use glutin::ContextTrait;
   |     ^^^^^^^^^^^^^^^^^^^^ no `ContextTrait` in the root. Did you mean to use `ContextError`?

warning: unused import: `glutin::ContextTrait`
  --> src/gl/mod.rs:16:5
   |
16 | use glutin::ContextTrait;
   |     ^^^^^^^^^^^^^^^^^^^^
   |
   = note: #[warn(unused_imports)] on by default

warning: unused import: `glutin::ContextTrait`
  --> src/main.rs:55:5
   |
55 | use glutin::ContextTrait;
   |     ^^^^^^^^^^^^^^^^^^^^

error[E0107]: wrong number of type arguments: expected 1, found 0
   --> src/main.rs:358:37
    |
358 | fn handle_window_event(window: &mut glutin::WindowedContext,
    |                                     ^^^^^^^^^^^^^^^^^^^^^^^ expected 1 type argument

error[E0107]: wrong number of type arguments: expected 1, found 0
  --> src/gl/mod.rs:25:20
   |
25 | pub fn init(vid: & glutin::WindowedContext) {
   |                    ^^^^^^^^^^^^^^^^^^^^^^^ expected 1 type argument

error: aborting due to 4 previous errors

Some errors occurred: E0107, E0432.
For more information about an error, try `rustc --explain E0107`.
error: Could not compile `stevenarella`.

[iceiix]

Removing the context trait imports is easy enough, but how to resolve the missing type arguments on WindowedContext? Added <T>, the compiler suggests adding where T: glutin::context::ContextCurrentState bounds but module context is private. (fixed: re-exported as glutin::ContextCurrentState) This is part of the "new context currency system":

• Added two new types, NotCurrent and PossiblyCurrent, which RawContext, WindowedContext and Context are now generic over.
• ContextBuilder is also generic over those two types.
• Which type it’s generic over depends on the currency state of the context you intend to share it with, not, as some have mistakenly thought, the currency in which the new context will be made. The new context will always be NotCurrent.

https://docs.rs/glutin/0.21.0/glutin/type.WindowedContext.html
https://docs.rs/glutin/0.21.0/glutin/struct.ContextWrapper.html

https://github.com/rust-windowing/glutin/blob/2662698ab872c8c56d052fb920baa6b854328293/glutin/src/windowed.rs#L98

impl<T: ContextCurrentState> WindowedContext<T> {