Change size_t to std::size_t, remove void from function parameters, use
constants to represent the minimum required number of control lines, and
change const auto & to const auto.
Set the "apply" and "delete all" button sensitivity based on the number
of control lines. Add extra tooltip text to the apply button when
editing and there is not enough lines. The text explains there must be
enough vertical or horizontal control lines.
When exiting perspective control line editing mode while dragging a
control line, set the geometry and control line state to the appropriate
state corresponding to when lines are not being dragged.
