A cheap trick i went with after i saw that there were no rational (direct, proportional to the goal) solution for this kind of thing (with unity i mean) here: https://www.sugarsync.com/pf/D2307301_98582733_612566
From the get-go you will see that it is possible to get ride of one sprite if you want to, by pushing the 3 remaining parts in a different order depending on the prc to display
Thanks! 
There are terrible ideas I came with in my first years with Unity and this is one of them.
Absolutely do never use the crap I linked here when starting this thread some ages ago.
If you really have no other choice, when creating Rad Filled Sprites for IMGUI (they already exist for non-editor UI) prefer something like this:
The explanations of the IMGUI Rad Filled Sprites is here:
The Sprite Demo for IMGUI Rad Filled Sprites is in this project:
The Video of what IMGUI Rad Filled Sprites gives is here:
The more triangles, the less deformation and more precise you are but 4 are generally sufficients.