Warn when ambient shadows are slowing down graphics

[Tom Goddard]

When moving an atomic model with the mouse while keeping other models fixed such as in fitting into an EM map, the graphics update can be painfully slow if full or soft lighting are used because the ambient shadows are recomputed every frame for the new model positions. It would be helpful to warn the user and suggest switching to simple lighting.

Maybe if the frame rate drops below some level e.g. 5 frames per second, and shadow updates are done on 10 or more consecutive frames, and the warning has not been issued in this same session already, then show in red in the log a warning message "ChimeraX is drawing the scene slowly because ambient shadows are being recomputed frequently. Use simple lighting (from home toolbar icon, or command "light simple") to get faster rendering". Here the light simple can be a command link.

This might be a bit annoying to see when recording a movie, but maybe it is ok since it only appears once.

Problem described by Pranav Shah on mailing list

​https://www.rbvi.ucsf.edu/pipermail/chimerax-users/2021-August/002509.html