Fix video export on macOS

Exporting video currently fails on macOS with the following error:

No provider of glNamedRenderbufferStorageMultisample found.  Requires one of:
    Desktop OpenGL 4.5
    GL_ARB_direct_state_access
[1]    98116 abort      robofish-trackviewer --draw-labels track_000.hdf5

macOS 10.14.6 (Mojave)

robofish-trackviewer==0.6.1