overlay: create a dedicated thread for polling hotkeys (#864)

## Link to GitHub Issue or related Pull Request, if one exists
n/a

## Description of change
Create a new thread that polls for the following hotkeys:

* super exit (both alt+f4 and bound key)
* coin insert
* screenshot

The goal is to make the capture of these more reliable, because before
this PR it wasn't.

## Testing

src/spice2x/hooks/graphics/graphics.cpp

@@ -17,11 +17,13 @@
 #include "games/ddr/ddr.h"
 #include "games/gitadora/gitadora.h"
 #include "games/iidx/iidx.h"
+#include "games/io.h"
 #include "games/sdvx/sdvx.h"
 #include "games/popn/popn.h"
 #include "hooks/graphics/backends/d3d9/d3d9_backend.h"
 #include "hooks/graphics/backends/d3d11/d3d11_backend.h"
 #include "launcher/shutdown.h"
+#include "misc/hotkeys.h"
 #include "overlay/overlay.h"
 #include "touch/touch.h"
 #include "touch/touch_gestures.h"
@@ -1374,6 +1376,12 @@ void graphics_screens_get(std::vector<int> &screens) {
     screens.insert(screens.end(), GRAPHICS_SCREENS.begin(), GRAPHICS_SCREENS.end());
 }
 
+void graphics_poll_screenshot_hotkey() {
+    if (hotkeys::consume_screenshot()) {
+        graphics_screenshot_trigger();
+    }
+}
+
 void graphics_screenshot_trigger() {
     GRAPHICS_SCREENSHOT_TRIGGER = true;
 }