DolphinQt: Rework TAS input threading, part 1 (buttons)

This gets rid of a blocking operation, improving performance and fixing
https://bugs.dolphin-emu.org/issues/12893.

This also makes us no longer directly access the state of certain UI
elements from the CPU thread, which probably wasn't thread-safe but
doesn't seem to have caused any observable issues so far.

Source/Core/DolphinQt/TAS/TASControlState.cpp

@@ -0,0 +1,58 @@
+// Copyright 2023 Dolphin Emulator Project
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+#include "DolphinQt/TAS/TASControlState.h"
+
+#include <atomic>
+
+#include "Common/CommonTypes.h"
+
+u16 TASControlState::GetValue() const
+{
+  const State ui_thread_state = m_ui_thread_state.load(std::memory_order_relaxed);
+  const State cpu_thread_state = m_cpu_thread_state.load(std::memory_order_relaxed);
+
+  return (ui_thread_state.version != cpu_thread_state.version ? cpu_thread_state : ui_thread_state)
+      .value;
+}
+
+bool TASControlState::OnControllerValueChanged(u16 new_value)
+{
+  const State cpu_thread_state = m_cpu_thread_state.load(std::memory_order_relaxed);
+
+  if (cpu_thread_state.value == new_value)
+  {
+    // The CPU thread state is already up to date with the controller. No need to do anything
+    return false;
+  }
+
+  const State new_state{static_cast<u16>(cpu_thread_state.version + 1), new_value};
+  m_cpu_thread_state.store(new_state, std::memory_order_relaxed);
+
+  return true;
+}
+
+void TASControlState::OnUIValueChanged(u16 new_value)
+{
+  const State ui_thread_state = m_ui_thread_state.load(std::memory_order_relaxed);
+
+  const State new_state{ui_thread_state.version, new_value};
+  m_ui_thread_state.store(new_state, std::memory_order_relaxed);
+}
+
+u16 TASControlState::ApplyControllerValueChange()
+{
+  const State ui_thread_state = m_ui_thread_state.load(std::memory_order_relaxed);
+  const State cpu_thread_state = m_cpu_thread_state.load(std::memory_order_relaxed);
+
+  if (ui_thread_state.version == cpu_thread_state.version)
+  {
+    // The UI thread state is already up to date with the CPU thread. No need to do anything
+    return ui_thread_state.value;
+  }
+  else
+  {
+    m_ui_thread_state.store(cpu_thread_state, std::memory_order_relaxed);
+    return cpu_thread_state.value;
+  }
+}