Viewports: Backported 'void* ImGuiViewport::PlatformHandle' from docking branch for use by backends.

imgui.h

@@ -2202,7 +2202,7 @@ struct ImGuiIO
     // - Backends may have other side-effects on focus loss, so this will reduce side-effects but not necessary remove all of them.
     bool        ConfigDebugIgnoreFocusLoss;     // = false          // Ignore io.AddFocusEvent(false), consequently not calling io.ClearInputKeys()/io.ClearInputMouse() in input processing.
 
-    // Options to audit .ini data
+    // Option to audit .ini data
     bool        ConfigDebugIniSettings;         // = false          // Save .ini data with extra comments (particularly helpful for Docking, but makes saving slower)
 
     //------------------------------------------------------------------
@@ -3240,6 +3240,7 @@ struct ImGuiViewport
     ImVec2              WorkSize;               // Work Area: Size of the viewport minus task bars, menu bars, status bars (<= Size)
 
     // Platform/Backend Dependent Data
+    void*               PlatformHandle;         // void* to hold higher-level, platform window handle (e.g. HWND, GLFWWindow*, SDL_Window*)
     void*               PlatformHandleRaw;      // void* to hold lower-level, platform-native window handle (under Win32 this is expected to be a HWND, unused for other platforms)
 
     ImGuiViewport()     { memset(this, 0, sizeof(*this)); }