Made default clipboard handlers for Win32 and OSX use a buffer inside the main context instead of a static buffer, so it can be freed properly on Shutdown. (#3110)

imgui_internal.h

@@ -1189,7 +1189,7 @@ struct ImGuiContext
     float                   DragSpeedDefaultRatio;              // If speed == 0.0f, uses (max-min) * DragSpeedDefaultRatio
     float                   ScrollbarClickDeltaToGrabCenter;    // Distance between mouse and center of grab box, normalized in parent space. Use storage?
     int                     TooltipOverrideCount;
-    ImVector<char>          PrivateClipboard;                   // If no custom clipboard handler is defined
+    ImVector<char>          ClipboardHandlerData;               // If no custom clipboard handler is defined
     ImVector<ImGuiID>       MenusIdSubmittedThisFrame;          // A list of menu IDs that were rendered at least once
 
     // Platform support