fix: screenchar()/screenstring() with multigrid #32494

Problem:
- When multigrid is enabled, screenchar()/screenstring() functions return wrong
  results. See https://github.com/neovide/neovide/issues/2569
- `screenstring()` executed via RPC in child Nvim process, doesn't recognize
  floating windows.

Solution:
In ui_comp_get_grid_at_coord(), also iterate window grids.

src/nvim/ui_compositor.c

@@ -333,6 +333,14 @@ ScreenGrid *ui_comp_get_grid_at_coord(int row, int col)
       return grid;
     }
   }
+
+  FOR_ALL_WINDOWS_IN_TAB(wp, curtab) {
+    ScreenGrid *grid = &wp->w_grid_alloc;
+    if (row >= grid->comp_row && row < grid->comp_row + grid->rows
+        && col >= grid->comp_col && col < grid->comp_col + grid->cols) {
+      return grid;
+    }
+  }
   return &default_grid;
 }