service/vi/vi_layer: Convert Layer struct into a class

Like the previous changes made to the Display struct, this prepares the
Layer struct for changes to its interface. Given Layer will be given
more invariants in the future, we convert it into a class to better
signify that.

src/core/hle/service/nvflinger/nvflinger.h

@@ -29,7 +29,7 @@ class Module;
 
 namespace Service::VI {
 class Display;
-struct Layer;
+class Layer;
 } // namespace Service::VI
 
 namespace Service::NVFlinger {