Improve rounded border corners

[WillPower3309]

• Description: There are some improvements that could be made in how we handle rendering rounded corners. Currently, rounded borders are done by drawing circles and removing the parts that are not along the corner we are currently rendering, and rounded titlebars will be blended with a window's rounded corners through its fragment shader, which is less than ideal for keeping things simple. I'll be working on making these things cleaner (and hopefully more performant), once we get an initial release out.