On recent Mac systems (Catalina) OpenGL is slower compared to
directly using GTK's NSView. Furthermore when compiled on
Catalina, and running on a system with a Retina screen, the initial
size of the canvas may be wrong:
libs/gtkmm2ext/nsglview.mm:175:1: warning: method possibly
missing a [super reshape] call [-Wobjc-missing-super-calls]
This is not an issue when running versions compiled on older systems.