Added support for gradients as background variants + other optimizations.

style/src/button.rs

@@ -68,6 +68,9 @@ pub trait StyleSheet {
                     a: color.a * 0.5,
                     ..color
                 }),
+                Background::Gradient(gradient) => {
+                    Background::Gradient(gradient.mul_alpha(0.5))
+                }
             }),
             text_color: Color {
                 a: active.text_color.a * 0.5,