st/scroll-view: Ensure float are used explictly

Pineview needs explicit float to prevent error when shader is compiled.

Closes: #4127
Part-of: <!1815>

src/st/st-scroll-view-fade.glsl

@@ -70,6 +70,6 @@ void main ()
 
         cogl_color_out *= ratio;
     } else {
-        cogl_color_out *= 0;
+        cogl_color_out *= 0.0;
     }
 }