Win32: Fix disabled cursor mode vs caption buttons

This postpones disabling the cursor until the user is done interacting
with a caption button.

Related to #650.
Fixes #1071.

src/win32_platform.h

@@ -233,6 +233,7 @@ typedef struct _GLFWwindowWin32
     HICON               smallIcon;
 
     GLFWbool            cursorTracked;
+    GLFWbool            frameAction;
     GLFWbool            iconified;
     GLFWbool            maximized;