subcompositor/renderer: fixup handling of subsurfaces below the main one

some apps (notably vlc 4) place a subsurface below the main surface (which is kinda cursed) but we have to accomodate for that

src/protocols/core/Subcompositor.hpp

@@ -35,6 +35,8 @@ class CWLSubsurfaceResource : public ISurfaceRole {
 
     WP<CWLSubsurfaceResource> self;
 
+    int                       zIndex = 1; // by default, it's above
+
     struct {
         CSignal destroy;
     } events;