Internals: storing HoveredWindowBeforeClear for use by multi-context compositor drag and drop propagation.

# Conflicts:
#	imgui.cpp
#	imgui_internal.h

imgui.cpp

@@ -4571,6 +4571,7 @@ void ImGui::UpdateHoveredWindowAndCaptureFlags()
     // - We also support the moved window toggling the NoInputs flag after moving has started in order to be able to detect windows below it, which is useful for e.g. docking mechanisms.
     bool clear_hovered_windows = false;
     FindHoveredWindowEx(g.IO.MousePos, false, &g.HoveredWindow, &g.HoveredWindowUnderMovingWindow);
+    g.HoveredWindowBeforeClear = g.HoveredWindow;
 
     // Modal windows prevents mouse from hovering behind them.
     ImGuiWindow* modal_window = GetTopMostPopupModal();

imgui_internal.h

@@ -1959,6 +1959,7 @@ struct ImGuiContext
     ImGuiWindow*            CurrentWindow;                      // Window being drawn into
     ImGuiWindow*            HoveredWindow;                      // Window the mouse is hovering. Will typically catch mouse inputs.
     ImGuiWindow*            HoveredWindowUnderMovingWindow;     // Hovered window ignoring MovingWindow. Only set if MovingWindow is set.
+    ImGuiWindow*            HoveredWindowBeforeClear;           // Window the mouse is hovering. Filled even with _NoMouse. This is currently useful for multi-context compositors.
     ImGuiWindow*            MovingWindow;                       // Track the window we clicked on (in order to preserve focus). The actual window that is moved is generally MovingWindow->RootWindow.
     ImGuiWindow*            WheelingWindow;                     // Track the window we started mouse-wheeling on. Until a timer elapse or mouse has moved, generally keep scrolling the same window even if during the course of scrolling the mouse ends up hovering a child window.
     ImVec2                  WheelingWindowRefMousePos;
@@ -2276,6 +2277,7 @@ struct ImGuiContext
         CurrentWindow = NULL;
         HoveredWindow = NULL;
         HoveredWindowUnderMovingWindow = NULL;
+        HoveredWindowBeforeClear = NULL;
         MovingWindow = NULL;
         WheelingWindow = NULL;
         WheelingWindowStartFrame = WheelingWindowScrolledFrame = -1;