Make rounded corners clip the image in ImageButton

[Kesanov]

Currently, the image remains in its original shape - usually rectangular.

Furthermore, the border is painted under the image.
If it had been painted on top, it would appear to clip the image beneath.

[Kesanov]

Additionally, the border seems to be few pixels wider than it should:

[emilk]

Making a frame that looks good for any image is difficult, but I at least made some improvements.

Rounded corners on an egui::Image and egui::ImageButton would be super nice, and not that difficult to implement if someone wants to take a stab at it.