SameLine() with explicit X position is relative to left of group/columns (ref #746, #125, #630)

imgui_internal.h

@@ -546,6 +546,7 @@ struct IMGUI_API ImGuiDrawContext
     int                     StackSizesBackup[6];    // Store size of various stacks for asserting
 
     float                   IndentX;                // Indentation / start position from left of window (increased by TreePush/TreePop, etc.)
+    float                   GroupOffsetX;
     float                   ColumnsOffsetX;         // Offset to the current column (if ColumnsCurrent > 0). FIXME: This and the above should be a stack to allow use cases like Tree->Column->Tree. Need revamp columns API.
     int                     ColumnsCurrent;
     int                     ColumnsCount;