Cocoa: Filter out duplicate size events

Fixes #1085.

src/cocoa_platform.h

@@ -88,6 +88,10 @@ typedef struct _GLFWwindowNS
 
     GLFWbool        maximized;
 
+    // Cached window and framebuffer sizes used to filter out duplicate events
+    int             width, height;
+    int             fbWidth, fbHeight;
+
     // The total sum of the distances the cursor has been warped
     // since the last cursor motion event was processed
     // This is kept to counteract Cocoa doing the same internally