tui: eliminate scrolling region data structure

The scrolling region is always local to a single grid_scroll event, use
local variables and parameters instead.

The invocation of reset_scroll_region in grid_resize is cargo-culted to use
margin reset under exactly the same circumstances, not sure if it is necessary
though.

src/nvim/ugrid.h

@@ -15,7 +15,6 @@ struct ucell {
 };
 
 struct ugrid {
-  int top, bot, left, right;
   int row, col;
   int width, height;
   UCell **cells;