Graph — Flood Fill

A grid of colors is a graph in disguise: each cell is a node connected to its 4
neighbours (up, down, left, right — not diagonals).

solve(image, sr, sc, color) — starting at cell (sr, sc), recolor that cell
and every cell connected to it through cells of the same original color.
Return the new image; do not mutate the input.

Examples

solve([[1,1,1],[1,1,0],[1,0,1]], 1, 1, 2)  → [[2,2,2],[2,2,0],[2,0,1]]
solve([[0,0,0],[0,0,0]], 0, 0, 0)          → [[0,0,0],[0,0,0]]   // already that color
solve([[1,2],[2,1]], 0, 0, 3)              → [[3,2],[2,1]]       // no diagonals

If the new color equals the original one, return early — otherwise a naive
search never marks anything as visited and loops forever.