compositing: Split out Painter from IOCompositor (servo#40264)

This change splits `IOCompositor` into two structs: `IOCompositor` and
`Painter`. The idea is that `Painter` will be per-`RenderingContext`.
Currently there is only one, but when there is more than one there will
be one `Painter` per-`RenderingContext`. Therefore, everything that we
suspect will be managed this way is moved to `Painter`.

In addition, to avoid the risk of double-borrows, `WebViewRender` no
longer keeps a reference to any of the containing structs. Instead
necessary things are passed down when necessary or stored internally.

Testing: This should not change behavior and is thus covered by existing
tests.
Fixes: This is part of servo#40261.

Signed-off-by: Martin Robinson <[email protected]>
Co-authored-by: Mukilan Thiyagarajan <[email protected]>

Author: mrobinson

Cargo.lock

@@ -47,6 +47,7 @@ servo-tracing = { workspace = true }
 servo_allocator = { path = "../allocator" }
 servo_config = { path = "../config" }
 servo_geometry = { path = "../geometry" }
+smallvec = { workspace = true }
 stylo_traits = { workspace = true }
 timers = { path = "../timers" }
 tracing = { workspace = true, optional = true }