Fix stale rendering and shutdown race on terminal resize

[reisub]

Hey there, thanks for the great lib - definitely fills a gap in the Elixir world!

I'm working on a TUI app and I ran into an issue where resizing the terminal window doesn't correctly re-render the UI if there hasn't been an update after the resize.

I'm not super familiar with the codebase, so this is a Claude assisted fix which I've iterated on and confirmed that it fixes the issue I ran into.

Changes:

Drain the message queue between broadcasting Resize and rendering, so the first frame after a resize reflects updated component state instead of the previous frame's.

Replace the unconditional backend.clear with the diff renderer's erase cells when a buffer_manager is in play, falling back to a real clear for buffer-less backends.

Skip the post-resize render if a component's update returned :quit so we don't paint into a backend that's about to be torn down.