gui2/tcustom_tod: Remove direct call to gui2::twindow::draw()

It's unnecessary, results in a noticeable performance penality when
updating ToD color shift sliders, and the documentation says it's not to
be called directly (which raises the question of why it's a public
method in the first place).
gui2/tcustom_tod: Avoid having display::draw() commit the framebuffer

This allows the dialog to be redrawn on top of the updated display
contents once we are done without any flickering, and performance is
much better because this results in the framebuffer being committed once
instead of twice every time ToD color shift slider events are processed.