render

type Grid interface {
	NumRows() int
	NumCols() int
	// LetterAt returns the letter at the given row and column.
	// Returning '.' will cause the square to render black.
	// Returning ' ' or '-' or 0 will cause no letter to be rendered.
	LetterAt(row, col int) byte
	// ShadeAt returns the shade in range [0,1] to shade the square at the given row/column.
	// It will only be called for white squares with no letters.
	ShadeAt(row, col int) float64
}