rawinput: fix spicecfg close delay (#715)

## Link to GitHub Issue or related Pull Request, if one exists
#0

## Description of change
Closing spicecfg was blocked for up to ~495 ms. Runtime behavior is
unchanged: same flush interval and same output path during normal
operation. Only shutdown teardown is faster.

## Testing
Opened and closed spicecfg.exe repeatedly; the window closes immediately
with no hang

src/spice2x/rawinput/rawinput.h

@@ -69,6 +69,8 @@ namespace rawinput {
         std::thread *input_thread = nullptr;
         std::thread *flush_thread = nullptr;
         bool flush_thread_running = false;
+        std::mutex flush_thread_m;
+        std::condition_variable flush_thread_cv;
         std::thread *output_thread = nullptr;
         std::mutex output_thread_m;
         bool output_thread_ready = false;