Fix texture sharing on Windows nVidia systems
Merge pull request #99 from simonster/texture-sharing

Fix texture sharing on nVidia Windows systems:

"Theory" of why it failed and now works with this patch applied:

wglShareLists according to doc should reject sharing if the context which is going to share resources already contains any resources. This is not the case in our code. But the context already has sharing enabled with other new context which also don't contain any resources to share yet. So either the NVidia driver has a bug and uses a too sloppy way to detect if context sharing is allowed - implying that a context who shares resources with other contexts must already contain resources, without actually checking if this exclusion criterion is really satisfied - or the Microsoft docu is sloppily written and gives wrong sharing rules. Reordering the code like you do should "fix" both possible cases.