graphics: option to include subscreens in screenshot (#866)

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

## Description of change
Adds `-screenshotsub`. Off by default. When on, screenshots include the
subscreens. All subscreens are captured, even if they are hidden from
view. Works for all four screens of gitadora arena model as well.

This should also fix an issue with API / companion app not capturing
gitadora subscreen correctly.

Note: only done for DX9... DX11 will need another PR to make this work.

Unrelated to this PR, there seems to be a bug with gitadora not
accepting mouse or api touch input.

## Testing

src/spice2x/launcher/options.cpp

@@ -3405,6 +3405,14 @@ static const std::vector<OptionDefinition> OPTION_DEFINITIONS = {
         .type = OptionType::Bool,
         .category = "General Overlay",
     },
+    {
+        // ScreenshotSubscreens
+        .title = "Include Subscreens in Screenshots",
+        .name = "screenshotsub",
+        .desc = "Saves each subscreen as a separate PNG alongside the primary screenshot.",
+        .type = OptionType::Bool,
+        .category = "General Overlay",
+    },
 };
 
 const std::vector<std::string> &launcher::get_categories(Options::OptionsCategory category) {