Computes the vector coordinates, for the given x,y texture coordinates
and the given cube map face.
Also computes the solid angle for those coordinates and returns it.
To know what the solid angle is please read this.
Original solid angle calculation code is from Ignacio Castaño. This
formula is from Manne Öhrström's thesis. It takes two coordiantes in the
range [-1, 1] that define a portion of a cube face and return the area of
the projection of that portion on the surface of the sphere.