DOC: ImageGL needs WebGL

[pllim]

From testing #37 , I found that ImageGL does not work (no error message, just nothing shows) on RHEL 7 (has NVidia but the driver is probably very old). Unfortunately, it is a company machine and I am unable to upgrade anything there.

This WebGL requirement should be properly documented.

[pllim]

For whatever reason, driver upgrade didn't fix the problem on Firefox but it worked on Chrome. 🤷

[astrofrog]

Is it enough to specify a minimum browser version for each browser or does the OS matter?

[astrofrog]

We could always link to https://get.webgl.org/ which has a test. There are also useful tables like https://docs.unity3d.com/Manual/webgl-browsercompatibility.html out there - I don't think we want to replicate all my this but could link to these kinds of tables.

@maartenbreddels do we require WebGL or WebGL 2? For the latter having a minimum browser version would be easy to state I think.

[maartenbreddels]

Only WebGL, https://webglreport.com/?v=1 is also nice in case we need particular features

[pllim]

Even just a note on README and installation (wherever you deem most appropriate) that says "check this first and see if you see a spinning cube" would have been nice. Also need similar note in jdaviz.

Even better if there is some kind of API to check this at start up and fails if WebGL isn't working.