Right now controls use a custom path to implement the CSS specification that allows the user to define different corner radii for each corner.  This makes it much harder to optimize the rendering of their backgrounds (and the effects that get applied to them).

Note that we currently only optimize a few aspects of rounded rectangles, but if they were enhanced so they could be the default background for the majority of controls then their optimizations would become more important.  Until we provide a specific primitive to handle this control background rendering case, though, we have very little chance of optimizing these operations for controls anyway so this is the first step towards control optimization.

This new capability does not need to be exposed via the JavaFX Rectangle{} node so this request does not necessarily involve any external API changes.