Columns: ImDrawList::Channels* functions now work inside columns.

Use a private splitter in columns, paving way for removal of obsolete ImDrawList::Channels* functions.

imgui.h

@@ -1987,8 +1987,11 @@ struct ImDrawList
     IMGUI_API ImDrawList* CloneOutput() const;                                  // Create a clone of the CmdBuffer/IdxBuffer/VtxBuffer.
 
     // Advanced: Channels
-    // - Use to split render into layers. By switching channels to can render out-of-order (e.g. submit foreground primitives before background primitives)
-    // - Use to minimize draw calls (e.g. if going back-and-forth between multiple non-overlapping clipping rectangles, prefer to append into separate channels then merge at the end)
+    // - Use to split render into layers. By switching channels to can render out-of-order (e.g. submit FG primitives before BG primitives)
+    // - Use to minimize draw calls (e.g. if going back-and-forth between multiple clipping rectangles, prefer to append into separate channels then merge at the end)
+    // - FIXME-OBSOLETE: This API shouldn't have been in ImDrawList in the first place!
+    //   Prefer using your own persistent copy of ImDrawListSplitter as you can stack them.
+    //   Using the ImDrawList::ChannelsXXXX you cannot stack a split over another.
     inline void     ChannelsSplit(int count)    { _Splitter.Split(this, count); }
     inline void     ChannelsMerge()             { _Splitter.Merge(this); }
     inline void     ChannelsSetCurrent(int n)   { _Splitter.SetCurrentChannel(this, n); }