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/launcher/options.h

@@ -320,7 +320,8 @@ namespace launcher {
             OBSWebSocketHost,
             OBSWebSocketPort,
             OBSWebSocketPassword,
-            OBSWebSocketDebug
+            OBSWebSocketDebug,
+            ScreenshotIncludeOverlay
         };
 
         enum class OptionsCategory {