Add the ability invert the X and/or Y axis

[CattleProdigy]

By "invert" I mean to change the default direction of the axis. The standard convention for plots is for +X to go to the right and +Y to go up. In some scenarios, particularly working with images or camera coordinates, +Y should point down.

There's already an existing strategy for inverting the Y axis in the PlotTransform struct, since screen coordinates have +Y pointing down and plots conventionally have +Y pointing up. That strategy was expanded upon and made configurable from Plot.

A few questions:

• I avoided breaking PlotTransform::new by providing PlotTransform::new_with_invert_axis which is a bit cumbersome to write out. Would we rather break the API by just adding a parameter to new?
• I didn't expect there to be any differences from the test snapshot, but there were several small ones. Most of the changes can't really be seen, though there are a few pixels with apparent differences. I don't know if this is just platform differences. The code changes are simple enough that for the not-inverted case, I don't see how anything really out to change. Assuming these are just platform differences, is the convention to update the screenshots or leave it?

• Closes egui_plot invert y axis #21
• I have followed the instructions in the PR template

[bircni]

could you also add an example to the demo?

[CattleProdigy]

could you also add an example to the demo?

Done here: f36888e

[bircni]

lgtm