When using proportional editing in the Graph Editor or the Dope Sheet,
the GUI displays a circle suggesting that the influence is limited in x and y.
In reality those editors only look at the x-axis to determine which keys to influence.
This PR fixes that by adding a new drawing function for those editors that
draws 2 lines to indicate a range.
In theory this can be a feature in other 2D editors in the future.
I assume the VSE would benefit from this as well.
Pull Request: https://projects.blender.org/blender/blender/pulls/118196
