Revert back to the standard OpenGL namespace

Turns out using the OpenGL4 namespace is okay within the library itself, but it forces any project referencing the library to use it as well, which is not a good idea.
Fix multiple calls to GLResource.Dispose() resulting in OpenGL errors
Add factory method to create a Texture2D instance on an already existing texture handle
Add additional shader property mappings

Remove the requirement of a VertexAttribAttribute on all VertexAttrib properties by supplying defaults when no attribute is tagged.
Add advanced example for parallax mapping

Refactor OpenTK example for parallax mapping to show off some DerpGL usage