[gpu] Fix infinite hangup/freezes at shutdown/reset (#4395)

Thread may be destroyed at dtor(), but it hasn't fully shut down, so before notify shutdown, request immediate stop (effective immediately).

Like the issue was that the thread didn't want to stop running, thus it would hang, it would also reference objects which were being destroyed without waiting for them to actually be destroyed
So it would reference invalid data
This PR tells the thread: "hey, STOP now, and DESTROY yourself"
So all of that should be avoided

Signed-off-by: lizzie <lizzie@eden-emu.dev>

- [x] I have read and followed the [Contribution Guidelines](https://git.eden-emu.dev/eden-emu/eden/src/branch/master/CONTRIBUTING.md#code-contributions).
- [x] I have read and followed the [AI Policy](https://git.eden-emu.dev/eden-emu/eden/src/branch/master/docs/policies/AI.md)
- [x] I have read and followed the [Coding Guidelines](https://git.eden-emu.dev/eden-emu/eden/src/branch/master/docs/policies/Coding.md) to the best of my ability.

-------------------

Reviewed-on: https://git.eden-emu.dev/eden-emu/eden/pulls/4395
Reviewed-by: Maufeat <sahyno1996@gmail.com>
Reviewed-by: MaranBr <maranbr@eden-emu.dev>

src/video_core/gpu_thread.cpp

@@ -114,4 +114,11 @@ u64 ThreadManager::PushCommand(CommandData&& command_data, bool block, bool is_a
     return fence;
 }
 
+void ThreadManager::NotifyShutdown() {
+    if (thread.joinable()) {
+        thread.request_stop();
+        thread.join();
+    }
+}
+
 } // namespace VideoCommon::GPUThread