MAINT: Add config var for opt-in

[larsoner]

From talking to @marsipu and @hoechenberger and running some OpenGL tests, we decided using OpenGL with the PyQtGraph backend should be opt-in. (TL;DR: Maintenance burden; OpenGL totally breaks things on my macOS machine; it's slower than CPU on my Linux machine with a good graphics card.) To facilitate this, we should have a MNE_BROWSE_USE_OPENGL config var, as it makes sense generally to have this set on a machine-by-machine basis / shouldn't really need to be set often (at all?) on a script-by-script basis.

Once this is merged, I can make a PR to mne-qt-browser to change the behavior:

1. Only try to use OpenGL if MNE_BROWSE_USE_OPENGL=true (otherwise use CPU)
2. If requested, then try importing PyOpenGL
3. If not found, emit a warning that it was requested but couldn't be used, and resort to CPU rendering

@drammock @agramfort feel free to merge if you agree with this plan