Acceder al elemento en [fila, columna].
Both row and column must be from 0 to 3 inclusive. A matrix is 4x4 array
of numbers, and you can access the individual elements using this function.
Tenga en cuenta la notación estándar de matemáticas - fila es el primer indice.
Accede al elemento en un indice sequencial (0..15 inclusivo).
A matrix is 4x4 array of numbers (16 numbers in total). You can access
the individual elements using "flattened" index with this.
The index is row+column*4.