Scoped binds

[nferhat]

I already spoke about this back in the matrix channel but I didn't have the time to actually try to implement this out.

The main idea is to give binds a lifetime, something similar to:

let mut renderer: GlesRenderer = todo!();

let tex1: GlesTexture = todo!();
let _scoped_bind: ScopedBind<'_, GlesRenderer, GlesTexture> = renderer.bind(tex1).unwrap();
{
    // In this scope, framebuffer is bound
    let framebuffer: GlesRenderbuffer = todo!();
    let _other_bind: ScopedBind<'_, GlesRenderer, GlesRenderBuffer> = renderer.bind(framebuffer).unwrap();
    // Now, the bind for framebuffer is dropped, and tex1 is bound again
}

Implementation is open to discussion

This is also needed as part of my rendering tests for blurring, as I need to (on Element::draw) bind a texture, render to it, and then bind back whatever I need to render to on the output surface. While accessing GlesRenderer::target would fix this issue for me, I'd rather implement this better interface too.

[nferhat]

The biggest difficulty is accessing the Renderer and the previous target when dropping a ScopedBind in order to bind it.

[nferhat]

First naive implementation looks kinda bad, not finished enough to get into a PR.

let mut bind = renderer.bind(buffer).map_err(Error::Rendering)?;
let renderer = bind.renderer().unwrap();
let res = self.render_output_internal(renderer, age, elements, clear_color)?;
bind.unbind().map(move |_| res).map_err(Error::Rendering)

Since the Drop trait doesn't have a return type, it has to be a separate function

[Drakulix]

See #1637

[nferhat]

See #1637

Mhm, seems to do exactly that

Well I guess this solves it, thank you!