feat: 添加VerticalSoften着色器，增强透明度控制逻辑

shaders/VerticalSoften.gdshader

@@ -0,0 +1,10 @@
+shader_type canvas_item;
+uniform vec2 percent=vec2(0,1);
+void fragment(){
+	COLOR=vec4(0,0,0,1);
+	if(UV.y<percent.x){
+		COLOR.a*=UV.y/percent.x;
+	}else if(UV.y>percent.y){
+		COLOR.a*=1.0-(distance(UV.y,percent.y))/(1.0-percent.y);
+	}
+}