mrview: lightbox: Reinitialise GL state when rendering each pane
mrview: lightbox: Reinitialise GL state when rendering each pane

* Fix for #1140
* However, this affects any rendered tool. Issue was that we need to ensure GL state is re-initialised because draw calls sequence is slice1, tools1, slice2, tools2 etc.
* Therefore, can't guarantee GL state remains unchanged between slices
* Ideally, want to ensure any tools restore original GL context at the end of their draw call, but regardless always safer to do this
MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3