Roof cell shading

A solar contractor models late afternoon shading on a flat roof. The roof arrives as a rectangular grid of cells, and heights[r][c] holds the height in centimetres of whatever stands on that cell, with 0 for bare deck and no negative values. The sun sits low on one side, so light crosses the roof in straight lines from the side named by light_from: 'W' runs it left to right along each row, 'E' right to left along each row, 'N' top to bottom down each column, and 'S' bottom to top up each column. A cell is shaded when some cell the light already passed on that same line stands strictly taller than it. Equal height clears. Return every shaded cell as a [row, column] pair, sorted by row and then by column. Every row has the same width.