Wayland: Clarify fallback decoration struct name

We now have three window decoration paths in the Wayland backend:
libdecor decorations, XDG decorations and fallback decorations.

This makes it clearer when code relates to the fallback decoration path,
which should increasingly (hopefully) not get used.

src/wl_platform.h

@@ -414,7 +414,7 @@ typedef struct _GLFWwindowWayland
         struct wl_buffer*                  buffer;
         _GLFWdecorationWayland             top, left, right, bottom;
         _GLFWdecorationSideWayland         focus;
-    } decorations;
+    } fallback;
 } _GLFWwindowWayland;
 
 // Wayland-specific global data