Matrix block sum

Given an m x n integer matrix mat and an integer k, return a matrix answer where answer[i][j] is the sum of all elements mat[r][c] with i-k <= r <= i+k and j-k <= c <= j+k, clamped to the matrix bounds. In other words, each output cell sums the (2k+1) x (2k+1) block centered on it, truncated at the edges. Values may be negative; the matrix has at least one row and column and k >= 0.