OpenGL: add checks for current context in debug mode
OpenGL: add checks for current context in debug mode

This adds checks for the GL context in the Texture, VertexBuffer,
VertexArrayObject, IndexBuffer, and FrameBuffer classes. These checks
verify that a context is current when these objects are deallocated, and
that this context matches the context that was current when they were
generated.

MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3