There is an `m x n` rectangular island that borders both the Pacific Ocean and Atlantic Ocean. The Pacific touches the island's top and left edges, and the Atlantic touches the bottom and right edges. The island is partitioned into a grid of square cells with heights. Rain water can flow to neighboring cells (up, down, left, right) if the neighboring cell's height is **less than or equal to** the current cell's height. Return a 2D list of grid coordinates `[r, c]` from which water can flow to **both** the Pacific and Atlantic oceans.