Take winit out of iris-core, which makes it build for Android

iris-core wanted exactly one thing from winit: PhysicalSize<u32> in
UiRenderNode::resize's signature, for two numbers it immediately turned
into floats. That pulled a whole windowing backend into the layer below
it. `resize` takes `impl Into<Vec2>` now, matching UiRenderState::resize
beside it.

The consequence is the reason: with winit in the graph, an Android build
of the core failed in android-activity, which needs a backend feature
nothing here selects and which iris should not be going through at all --
the plan is android-view. Without it, `cargo ndk -t arm64-v8a -P 26 build
-p iris-core` produces an rlib in 30s with wgpu's Android backend
included. So the widget, layout and render core already builds for the
phone, and what remains is the surface, the input and the IME.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

core/Cargo.toml

@@ -4,7 +4,6 @@ version.workspace = true
 edition.workspace = true
 
 [dependencies]
-winit = { workspace = true }
 wgpu = { workspace = true }
 bytemuck ={ workspace = true }
 image = { workspace = true }