use glew variable to check for the feature

Change-Id: I1e2ae37a75826d321268395d4a3c23c7ed8d7b17
let us assume that we need to special handling for mesa anymore

Change-Id: I37c4db8ad7283486dd8bb62b22342f6b127f0b08
that variable has nothing to do with glx

Change-Id: I202f716bcc65165e80928ccb81cec57f01bc0824
glew is setting up that function pointer for us

Change-Id: I1722c1d3816e6e995e71317a9c9b1529afe9df13