Reduced padding + unused storage in ImDrawList (224->192 bytes) + zero-init ImDrawListSplitter and ImDrawList

+ Readme tweak

imgui_internal.h

@@ -1692,9 +1692,9 @@ struct IMGUI_API ImGuiWindow
     ImGuiID                 NavLastIds[ImGuiNavLayer_COUNT];    // Last known NavId for this window, per layer (0/1)
     ImRect                  NavRectRel[ImGuiNavLayer_COUNT];    // Reference rectangle, in window relative space
 
-    bool                    MemoryCompacted;                    // Set when window extraneous data have been garbage collected
     int                     MemoryDrawListIdxCapacity;          // Backup of last idx/vtx count, so when waking up the window we can preallocate and avoid iterative alloc/copy
     int                     MemoryDrawListVtxCapacity;
+    bool                    MemoryCompacted;                    // Set when window extraneous data have been garbage collected
 
 public:
     ImGuiWindow(ImGuiContext* context, const char* name);