Restructure monitor enumeration

This way is both kinder on event-based enumeration and less work to
unwind allocations for when properly implementing GLFW_OUT_OF_MEMORY.

src/x11_platform.h

@@ -290,6 +290,7 @@ typedef struct _GLFWcursorX11
 } _GLFWcursorX11;
 
 
+void _glfwPollMonitorsX11(void);
 GLFWbool _glfwSetVideoModeX11(_GLFWmonitor* monitor, const GLFWvidmode* desired);
 void _glfwRestoreVideoModeX11(_GLFWmonitor* monitor);