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/graphics.h

@@ -111,6 +111,7 @@ extern bool FAKE_SUBSCREEN_ADAPTER;
 // settings
 extern std::string GRAPHICS_DEVICEID;
 extern std::string GRAPHICS_SCREENSHOT_DIR;
+extern bool GRAPHICS_SCREENSHOT_INCLUDE_OVERLAY;
 
 // Direct3D 9 settings
 extern std::optional<UINT> D3D9_ADAPTER;