ImDrawData: changed CmdLists from raw array to ImVector<> owned by ImDrawData itself. Faclitate user-manipulation of the array (#6406, #4879, #1878) + deep swap. (#6597, #6475, #6167, #5776, #5109, #4763, #3515, #1860)

+ Metrics: avoid misleadingly iterating all layers of DrawDataBuilder as everything is flattened into Layers[0] at this point.

# Conflicts:
#	imgui.cpp
#	imgui_internal.h

imgui_draw.cpp

@@ -1808,6 +1808,15 @@ void ImDrawListSplitter::SetCurrentChannel(ImDrawList* draw_list, int idx)
 // [SECTION] ImDrawData
 //-----------------------------------------------------------------------------
 
+void ImDrawData::Clear()
+{
+    Valid = false;
+    CmdListsCount = TotalIdxCount = TotalVtxCount = 0;
+    CmdLists.resize(0); // The ImDrawList are NOT owned by ImDrawData but e.g. by ImGuiContext, so we don't clear them.
+    DisplayPos = DisplaySize = FramebufferScale = ImVec2(0.0f, 0.0f);
+    OwnerViewport = NULL;
+}
+
 // For backward compatibility: convert all buffers from indexed to de-indexed, in case you cannot render indexed. Note: this is slow and most likely a waste of resources. Always prefer indexed rendering!
 void ImDrawData::DeIndexAllBuffers()
 {