Overhaul EmuWindow::PollEvents to fix yuzu-cmd calling SDL_PollEvents off main thread

EmuWindow::PollEvents was called from the GPU thread (or the CPU thread
in sync-GPU mode) when swapping buffers.  It had three implementations:

- In GRenderWindow, it didn't actually poll events, just set a flag and
  emit a signal to indicate that a frame was displayed.

- In EmuWindow_SDL2_Hide, it did nothing.

- In EmuWindow_SDL2, it did call SDL_PollEvents, but this is wrong
  because SDL_PollEvents is supposed to be called on the thread that set
  up video - in this case, the main thread, which was sleeping in a
  busyloop (regardless of whether sync-GPU was enabled).  On macOS this
  causes a crash.

To fix this:

- Rename EmuWindow::PollEvents to OnFrameDisplayed, and give it a
  default implementation that does nothing.

- In EmuWindow_SDL2, do not override OnFrameDisplayed, but instead have
  the main thread call SDL_WaitEvent in a loop.

src/core/frontend/emu_window.h

@@ -102,8 +102,8 @@ public:
         float render_surface_scale = 1.0f;
     };
 
-    /// Polls window events
-    virtual void PollEvents() = 0;
+    /// Called from GPU thread when a frame is displayed.
+    virtual void OnFrameDisplayed() {}
 
     /**
      * Returns a GraphicsContext that the frontend provides to be used for rendering.