Code review comment for lp:~vanvugt/mir/overlay-terminology-is-awkward

I wouldn't mind just calling it "overlay()" in future. However would prefer that be in future after we support the possibility of doing both in a single frame:

if (!display_buffer->overlay(the_lot))
{
   renderer->render(some)
   display_buffer->overlay(the_rest)
}

Remember "overlay" is a verb more often than it's a noun, so the "set_" prefix is unnecessary.