WebContext segfault on macOS

[jkelleyrtp]

Describe the bug
If the webcontext is dropped while the webview is still alive, there will be a segfault since the pointer lifetime is not properly tied to the webview.

We ran into this in Dioxus.

DioxusLabs/dioxus#935

The solution is to keep the pointer around, either internally via making webcontext owned instead of &mut or by leaking it.

Platform and Versions (please complete the following information):
OS: macOS
Rustc: 1.68.2

Would you want to assign yourself to resolve this bug?

• Yes
• No