Added missing GL context "Set Current" calls before loading data into vertex buffers.
Added missing GL context "Set Current" calls before loading data
into vertex buffers.

Changed behavior of Canvas3D::on_render() to postpone rendering to idle,
as on OSX the paint events happen directly from control updates,
if the control update triggers Canvas3D window rescaling. With this change,
rendering only happens from idle handler consistently.

prusa3d/Slic3r,prusa3d/Slic3r,prusa3d/Slic3r,prusa3d/Slic3r,prusa3d/Slic3r,prusa3d/Slic3r,prusa3d/Slic3r