[WIP] 0.21 Api changes.

[goddessfreya]

Compiles with the following backends:

• Linux | X11 | EGL
• Linux | X11 | GLX
• Linux | Wayland | EGL
• Linux | OSMesa
• Windows | WGL
• Windows | EGL
• Web | Emscripten
• macOS
• iOS
• Android

Has been tested with the following backends:

• Linux | X11 | EGL
• Linux | X11 | GLX
• Linux | Wayland | EGL
• Linux | OSMesa
• Windows | WGL
• Windows | EGL
• Web | Emscripten
• macOS
• iOS
• Android

For #501 and #784

[goddessfreya]

This PR comes with a slew of changes.

• The removal of libcaca: It is currently impossible for users to use libcaca (afaik), and it seams out of place for glutin.
• Contexts now take either PossiblyCurrentContext or NotCurrentContext as a generic parameter. The goal for this is to eventually implement Send + Sync on contexts which took NotCurrentContexts, but not on ones that took PossiblyCurrentContext. Contexts can be converted between the two via make_not_current and make_current.

Emphasis should be put on the "Possibly", however, because a user can just do this:

let c1: Context<PossiblyCurrentContext> = c1.make_current_context().unwrap();
let c2: Context<PossiblyCurrentContext> = c2.make_current_context().unwrap();

c1.get_proc_address("Uh oh, this is undefined.");

There is no way to stop this from happening, so we also expose treat_as_not_current, which in the example above would be called on c1 after making c2 current.

[goddessfreya]

https://gitlab.freedesktop.org/mesa/mesa/merge_requests/533