Wayland: Fix reporting of monitor scale changes

Content scale events would be emitted when a window surface entered or
left an output, but not when one of a window's current outputs had its
scale changed.

(cherry picked from commit e37ba80b13)

src/wl_platform.h

@@ -358,6 +358,7 @@ typedef struct _GLFWcursorWayland
 } _GLFWcursorWayland;
 
 void _glfwAddOutputWayland(uint32_t name, uint32_t version);
+void _glfwUpdateContentScaleWayland(_GLFWwindow* window);
 GLFWbool _glfwInputTextWayland(_GLFWwindow* window, uint32_t scancode);
 
 void _glfwAddSeatListenerWayland(struct wl_seat* seat);