tests/frustum: Add a few more points
simd4x4f: Add an init_frustum() initializer

Initializes a SIMD 4x4 matrix with the frustum described by six clipping
planes.
matrix: Add init_frustum()

The graphene_matrix_t wrapper around graphene_simd4x4f_init_frustum().

We can use it to initialize a matrix, and then use the matrix to
initialize a graphene_frustum_t.
tests/frustum: Add frustum matrix → frustum unit

Similar to the orthographic matrix → frustum unit.