Allowed drops from less privileged processes.

By default, UAC will prevent less privileged processes from sending
these messages.  This resolves that on Windows 7 and later.  Windows
Vista is still affected by this issue, as it has UAC but lacks
ChangeWindowMessageFilterEx.

Fixes #227.

src/win32_platform.h

@@ -77,6 +77,20 @@
 #ifndef WM_DWMCOMPOSITIONCHANGED
  #define WM_DWMCOMPOSITIONCHANGED 0x031E
 #endif
+#ifndef WM_COPYGLOBALDATA
+ #define WM_COPYGLOBALDATA 0x0049
+#endif
+#ifndef MSGFLT_ALLOW
+ #define MSGFLT_ALLOW 1
+#endif
+
+#if defined(__MINGW32__)
+typedef struct tagCHANGEFILTERSTRUCT
+{
+    DWORD cbSize;
+    DWORD ExtStatus;
+} CHANGEFILTERSTRUCT, *PCHANGEFILTERSTRUCT;
+#endif /*__MINGW32__*/
 
 
 //========================================================================
@@ -107,7 +121,9 @@ typedef DWORD (WINAPI * TIMEGETTIME_T) (void);
 
 // user32.dll function pointer typedefs
 typedef BOOL (WINAPI * SETPROCESSDPIAWARE_T)(void);
+typedef BOOL (WINAPI * CHANGEWINDOWMESSAGEFILTEREX_T)(HWND,UINT,DWORD,PCHANGEFILTERSTRUCT);
 #define _glfw_SetProcessDPIAware _glfw.win32.user32.SetProcessDPIAware
+#define _glfw_ChangeWindowMessageFilterEx _glfw.win32.user32.ChangeWindowMessageFilterEx
 
 // dwmapi.dll function pointer typedefs
 typedef HRESULT (WINAPI * DWMISCOMPOSITIONENABLED_T)(BOOL*);
@@ -192,6 +208,7 @@ typedef struct _GLFWlibraryWin32
     struct {
         HINSTANCE       instance;
         SETPROCESSDPIAWARE_T SetProcessDPIAware;
+        CHANGEWINDOWMESSAGEFILTEREX_T ChangeWindowMessageFilterEx;
     } user32;
 
     // dwmapi.dll