Method TableMergeDrawChannels

This function reorder draw channels based on matching clip rectangle, to facilitate merging them. Called by EndTable(). For simplicity we call it TableMergeDrawChannels() but in fact it only reorder channels + overwrite ClipRect, actual merging is done by table->DrawSplitter.Merge() which is called right after TableMergeDrawChannels(). Columns where the contents didn't stray off their local clip