First step towards #315 and #670

Things will change once pygfx refactors their viewport stuff, but I think it will be easier to adopt those changes after this PR since it makes Figure manage the bbox of all subplots. I think this is a good separation of responsibilities. After this PR arbitrary bboxes for subplots can be implemented.

• move the subplot viewport rect and subplot docks viewport rect calculating logic to Figure.
  • clean up the logic for computing the rects, much easier to read now, @clewis7 will be happy 😆
• Figure._render() and PlotArea._render() are now private, been meaning to do this for a long while.
• flatten cameras, controllers arrays in Figure to prep things for a flexible rectangles mode.
  • make sure all tests pass with this, adjust the tests if necessary. Users have a lot of options here and the parsing code is kinda gnarly.
• prepare Figure for flex rects mode by allowing Figure._subplots to be a 1D object array, figure methods have been adjusted accordingly.
• Figure.spacing is a new property which allows setting the spacing between subplots, default is 2 pixels
• add new screenshot test for gridplot with all subplot docks of a certain size, and an ImageWidget with all docks set to a certain size.
• adjust imgui toolbar code accordingly

Introduces one minor breaking API change, Subplot.position no longer exists but it's unlikely anyone was using that.

Todo in future PR:

• implement flexible resizable rectangle subplots
• make a separte plot area specifically for subplot titles, remove top subplot "dock" so users have less control over it. It should be solely for subplot title text.