[OpenTK] Make VB.Net overload resolution happy

GLHelper.cs contains a number of overloads that cannot be resolved by
VB.Net. We cannot remove them, as that will break
backward-compatibility, but we can hide them by moving them after the
autogenerated overloads in GL.cs.

Terrible hack, but it appears to work.

Issue originally reported at http://www.opentk.com/node/3554 as "OpenTK
1.1, VB.net and ambiguous functions."