NikolayNIK
TableSection layer and clip path caching
Right now _RenderTableSection repaints every layer and rebuilds the clip path on every repaint. There is a potential here to implement layer and clip path caching to futher improve the performance.
The idea is to add a bunch boolean flags to the RenderObject which would indicate which layers have to be repainted. On paint the only layers that are getting repainted are the ones which have the corresponding flag set and all flags get unset. Then add a bunch of named boolean arguments to the markNeedsRepaint function of the _RenderTableSection which would set the flags. It can not unset an already set flag. The only flag defaulting to true whould be a regular fixed layer one, since it is the only one that the widgets wrapping the row can paint on and they are the only ones which should call the method without arguments. To mark other layers for repaint the _RenderTableViewRow objects would have to trace the tree themselfs until they reach the _RenderTableSection and supply only needed flags. Since the use of repaint boundaries to wrap a row is already restricted, there should be no problem in skipping over other render objects (can even add an assert since the walk would be performed anyway).
We can go even futher and implement the same pattern with TableRow and TableCell widgets so that when the cell requires a repaint it would request a repaint of only a specific layer (e.g. either regular or clipped).
