As you can see the decagon is hardcoded. AFAIK there is no "draw circle"
command in OpenGL so the only way to get a smoother circle would be to
increase the numer of segments.
I'm surprised there is no type of primitive for arcs in OpenGL (or its
utility library GLU) so that hardware/CPU optimization on graphics card
drivers could be used.
The performance cost is the cos/sin computations, not the underlying
driver. The driver essentially sweeps out the same arc no matter how
fine the angular resolution is (i.e., it may draw more polygon lines,
but the individual polygon lines are shorter so don't need as many
computations to complete).
Rik's changeset works, e.g., test:
graphics_toolkit ('qt')
plot(rand(6,1), rand(6,1), 'or')
set(get(gca,'children'),'MarkerSize',100)
set(get(gca,'children'),'MarkerFaceColor','green')