Win32: Filter out duplicate size events

This mirrors the filtering done on X11 and Cocoa.  Possibly this should
be done by shared code instead.

Fixes #1610.

src/win32_platform.h

@@ -314,6 +314,9 @@ typedef struct _GLFWwindowWin32
     GLFWbool            scaleToMonitor;
     GLFWbool            keymenu;
 
+    // Cached size used to filter out duplicate events
+    int                 width, height;
+
     // The last received cursor position, regardless of source
     int                 lastCursorPosX, lastCursorPosY;
     // The last recevied high surrogate when decoding pairs of UTF-16 messages