Tables: separate bg0 and bg1 draw channels, selectable above bg color + will facilitate drawing of borders in EndTable().

+ unused code to strip out dummy draw channel calls.

imgui_internal.h

@@ -1890,7 +1890,7 @@ struct ImGuiTabBar
 
 #define IM_COL32_DISABLE                IM_COL32(0,0,0,1)   // Special sentinel code which cannot be used as a regular color.
 #define IMGUI_TABLE_MAX_COLUMNS         64                  // sizeof(ImU64) * 8. This is solely because we frequently encode columns set in a ImU64.
-#define IMGUI_TABLE_MAX_DRAW_CHANNELS   (1 + 2 + 64 * 2)    // See TableUpdateDrawChannels()
+#define IMGUI_TABLE_MAX_DRAW_CHANNELS   (4 + 64 * 2)        // See TableUpdateDrawChannels()
 
 // [Internal] sizeof() ~ 100
 // We use the terminology "Visible" to refer to a column that is not Hidden by user or settings. However it may still be out of view and clipped (see IsClipped).
@@ -2035,7 +2035,8 @@ struct ImGuiTable
     ImS8                        FreezeColumnsCount;         // Actual frozen columns count (== FreezeColumnsRequest, or == 0 when no scrolling offset)
     ImS8                        RowCellDataCurrent;         // Index of current RowCellData[] entry in current row
     ImU8                        DummyDrawChannel;           // Redirect non-visible columns here.
-    ImU8                        BgDrawChannelUnfrozen;      // Index within DrawSplitter.Channels[]
+    ImU8                        Bg1DrawChannelCurrent;      // For Selectable() and other widgets drawing accross columns after the freezing line. Index within DrawSplitter.Channels[]
+    ImU8                        Bg1DrawChannelUnfrozen;
     bool                        IsLayoutLocked;             // Set by TableUpdateLayout() which is called when beginning the first row.
     bool                        IsInsideRow;                // Set when inside TableBeginRow()/TableEndRow().
     bool                        IsInitializing;