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/launcher.cpp

@@ -1103,6 +1103,9 @@ int main_implementation(int argc, char *argv[]) {
     if (options[launcher::Options::ScreenshotFolder].is_active()) {
         GRAPHICS_SCREENSHOT_DIR = options[launcher::Options::ScreenshotFolder].value_text();
     }
+    if (options[launcher::Options::ScreenshotIncludeOverlay].value_bool()) {
+        GRAPHICS_SCREENSHOT_INCLUDE_OVERLAY = true;
+    }
     if (options[launcher::Options::DisableColoredOutput].value_bool()) {
         logger::COLOR = false;
     }