Patch submission: MultMatrix, LoadMatrix,etc. that accept ref Matrix4d

Hi, developers! First of all thank you for this great library! I have been using the latest SVN trunk to get access to the new Matrix4d and Vector4d types, but I found that GLHelper does not yet implement MultMatrix(ref Matrix4d) and friends yet (it does allow single precision "Matrix4" refs).

Here is a patch against GLHelper.cs that implements these routines. Not very complicated--just a cut and paste of the single precision Matrix4 routines with a change of type. I hope someone finds it useful, and feel free to integrate it into the trunk if you feel it adds value to the project.

Cheers,
Ryan