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

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