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/image_viewer/src/image_viewer.rs

@@ -155,7 +155,7 @@ impl FocusableView for ImageView {
 
 impl Render for ImageView {
     fn render(&mut self, cx: &mut ViewContext<Self>) -> impl IntoElement {
-        let checkered_background = |bounds: Bounds<Pixels>, _, cx: &mut ElementContext| {
+        let checkered_background = |bounds: Bounds<Pixels>, _, cx: &mut WindowContext| {
             let square_size = 32.0;
 
             let start_y = bounds.origin.y.0;