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/backends/d3d9/d3d9_screenshot.h

@@ -2,8 +2,6 @@
 
 #include <d3d9.h>
 
-void graphics_d3d9_poll_screenshot_hotkey();
-
 void graphics_d3d9_process_screenshot_and_capture(
         IDirect3DDevice9 *device,
         IDirect3DSwapChain9 *sub_swap_chain);