graphics: exclude overlay from screenshots (#865)

## Link to GitHub Issue or related Pull Request, if one exists
#0

## Description of change

Adds the `-screenshotoverlay` option.

By default, file screenshots exclude the Spice overlay. Enabling this
option captures screenshots after the overlay is rendered.

Also updates DX11 backend to allow screenshots even when overlay is
disabled.

## Testing

src/spice2x/hooks/graphics/backends/d3d9/d3d9_screenshot.h

@@ -2,6 +2,10 @@
 
 #include <d3d9.h>
 
-void graphics_d3d9_process_screenshot_and_capture(
+void graphics_d3d9_process_screenshot(
+        IDirect3DDevice9 *device,
+        IDirect3DSwapChain9 *sub_swap_chain);
+
+void graphics_d3d9_process_capture(
         IDirect3DDevice9 *device,
         IDirect3DSwapChain9 *sub_swap_chain);