renderer: Add supercircular window corners (#8943)

renderer: Add supercircular shadows and borders

config: Add rounding_power to default and example configs

rule: add `roundingpower` window rule

src/render/Shader.hpp

@@ -26,6 +26,7 @@ class CShader {
     GLint   fullSizeUntransformed = -1;
     GLint   radius                = -1;
     GLint   radiusOuter           = -1;
+    GLfloat roundingPower         = -1;
 
     GLint   thick = -1;