FloatUtil: Add in-place multMatrixf (since we mult line by line, we can multiply in place). Will be used by PMVMatrix.
Fix Bug 748 - PMVMatrix gluProject and gluUnProject broken w/ backingArray (default) ; Also: Use FloatUtil in-place multMatrixf.

PMVMatrix gluProject(..) and gluUnProject(..) don't pass their position (offset)
to the multMatrixf operation - even though they are sliced.

The API doc actually emphasizes this.