iOS: Fix UB in `Context::create_context`

[madsmtm]

We were calling an init method on the same object multiple times, which is not allowed in Objective-C.

Found while making #1440. With this change I can now run glutin on my old iPad.

• [x] Tested on all platforms changed
• [x] Compilation warnings were addressed
• [x] cargo fmt has been run on this branch
• [x] cargo doc builds successfully
• [x] Added an entry to CHANGELOG.md if knowledge of this change could be valuable to users
• [x] Updated documentation to reflect any user-facing changes, including notes of platform-specific behavior
• [x] Created or updated an example program if it would help users understand this functionality