add workaround

Yesterday a gl3n user on IRC found a DMD regression:
http://d.puremagic.com/issues/show_bug.cgi?id=11238

It affected Matrix.invert(); (wrong results), starting from DMD 2.063.

A simple program to expose the bug:

    import std.stdio;
    import gl3n.linalg;

    void main()
    {
        mat4 M = mat4(2, 0, 0, 0, 
                      0, 1, 0, 0,
                      0, 0, 1, 0,
                      0, 0, 0, 1);

        writefln("M = %s", M);
        M.invert();
        writefln("M = %s", M);
        M.invert();
        writefln("M = %s", M);
    }

This pull request is one way to workaround it. 
(damn, a newline in the diff)