setFromMatrixDirection

[mbredif]

With #14677, getWorldPosition and getWorldDirection have the same logic (except for the normalization of the direction) :

• call updateMatrixWorld( true )
• read a Vector3 from the matrixWorld

As a follow-up to #14677 (comment), this PR introduces setFromMatrixDirection as the counterpart of Vector3.setFromMatrixPosition, to be used directly when the matrixWorld is known to be up to date.

For instance, it would be useful in #14658 where light.matrixWorld is already synced :

three.js/src/renderers/webgl/WebGLLights.js

Line 205 in ddbdbaa

light.getWorldDirection( uniforms.direction ).negate();

Note: setFromMatrixDirection is equivalent to setFromMatrixColumn( m, 2 ).normalize() but I propose it anyway as it saves a few function calls and multiplications and is more explicit.

[WestLangley]

I don't think a matrix commonly has a 'direction'.

BTW, what you are doing is also equivalent to:

vector.set( 0, 0, 1 ).transformDirection( matrix );

which, for clarity I much prefer to:

var e = matrix.elements;
vector.set( e[ 8 ], e[ 9 ], e[ 10 ] ).normalize();

even though it is marginally slower.

And, as you said, it is also equivalent:

vector.setFromMatrixColumn( matrix, 2 ).normalize();

[mbredif]

Ok, I will then use the explicit vector.set( 0, 0, 1 ).transformDirection( matrix ); in my code and future PR whenever I know that the matrixWorld is in sync and getWorldDirection otherwise.
Thanks for the review.