Wayland: Fix fallback decoration cursor position

If fallback decorations were in use, pointer motion over a decoration
surface would cause glfwGetCursorPos to provide incorrect cursor
positions.

The cursor position is now only updated when the pointer is over the
content area of the window, similar to libdecor and XDG decorations.

src/wl_platform.h

@@ -413,6 +413,7 @@ typedef struct _GLFWwindowWayland
         struct wl_buffer*           buffer;
         _GLFWfallbackEdgeWayland    top, left, right, bottom;
         struct wl_surface*          focus;
+        wl_fixed_t                  pointerX, pointerY;
     } fallback;
 } _GLFWwindowWayland;