Merge ElementContext into WindowContext (#10979)

The new `ElementContext` was originally introduced to ensure the element
APIs could only be used inside of elements. Unfortunately, there were
many places where some of those APIs needed to be used, so
`WindowContext::with_element_context` was introduced, which defeated the
original safety purposes of having a specific context for elements.

This pull request merges `ElementContext` into `WindowContext` and adds
(debug) runtime checks to APIs that can only be used during certain
phases of element drawing.

Release Notes:

- N/A

---------

Co-authored-by: Nathan Sobo <nathan@zed.dev>

crates/gpui/src/elements/animation.rs

@@ -91,7 +91,7 @@ impl<E: IntoElement + 'static> Element for AnimationElement<E> {
 
     fn request_layout(
         &mut self,
-        cx: &mut crate::ElementContext,
+        cx: &mut crate::WindowContext,
     ) -> (crate::LayoutId, Self::RequestLayoutState) {
         cx.with_element_state(Some(self.id.clone()), |state, cx| {
             let state = state.unwrap().unwrap_or_else(|| AnimationState {
@@ -138,7 +138,7 @@ impl<E: IntoElement + 'static> Element for AnimationElement<E> {
         &mut self,
         _bounds: crate::Bounds<crate::Pixels>,
         element: &mut Self::RequestLayoutState,
-        cx: &mut crate::ElementContext,
+        cx: &mut crate::WindowContext,
     ) -> Self::PrepaintState {
         element.prepaint(cx);
     }
@@ -148,7 +148,7 @@ impl<E: IntoElement + 'static> Element for AnimationElement<E> {
         _bounds: crate::Bounds<crate::Pixels>,
         element: &mut Self::RequestLayoutState,
         _: &mut Self::PrepaintState,
-        cx: &mut crate::ElementContext,
+        cx: &mut crate::WindowContext,
     ) {
         element.paint(cx);
     }