gpui_windows: Give the window-create pointer mutable provenance

The WndProc recovers the CreateWindowExW lpParam as
*mut WindowCreateContext and writes through it, but the pointer was
derived from a shared reference, so it lacked mutable provenance.
Derive it from &raw mut instead.

crates/gpui_windows/src/window.rs

@@ -537,7 +537,10 @@ impl WindowsWindow {
                 parent_hwnd,
                 None,
                 Some(hinstance.into()),
-                Some(&context as *const _ as *const _),
+                // The `WndProc` recovers this as `*mut WindowCreateContext` and writes
+                // through it (setting `inner`), so the pointer must carry mutable
+                // provenance. Derive it from `&raw mut` rather than a shared reference.
+                Some(&raw mut context as *const _),
             )
         };