UIKit: preserve queued GPU redraws while waiting

[brianconnoly]

Summary

• allow GL/Metal-backed request_redraw calls to queue redraws while the iOS event loop is waiting, polling, or already processing redraws
• wake the CFRunLoop timer when queued GPU redraw work arrives
• re-arm the requested WaitUntil deadline after the GPU redraw queue drains, instead of leaving the repeating 0.1 microsecond timer hot
• keep immediate wakeups while GPU redraw work remains queued

The transition is expressed through a small waker-action helper. The regression test drained_gpu_queue_rearms_wait_until_deadline covers both the drained and still-queued paths.

Validation

• git diff --check
• cargo +nightly fmt --check -p winit-uikit
• cargo test -p winit-uikit --target aarch64-apple-ios --no-run (compile-level validation; the target binary cannot execute on the host)
• full upstream CI matrix passes for commit 488722b0