Fix compile warnings/errors in metal-cpp.h

Forward declare MTL::Drawable to avoid conflicts. "Drawable" is a common type name

renderdoc/driver/metal/official/metal-cpp.h

@@ -4511,6 +4511,7 @@ _MTL_PRIVATE_DEF_SEL(writeMask,
 
 namespace MTL
 {
+class Drawable;
 using DrawablePresentedHandler = void (^)(class Drawable*);
 
 using DrawablePresentedHandlerFunction = std::function<void(class Drawable*)>;