Fix click event with FlipVerticalTransformer

Credit come to the author of this commit :
ToxicBakery@f43fb3b

Author: Nico60

library/src/main/java/com/ToxicBakery/viewpager/transforms/FlipVerticalTransformer.java

@@ -30,4 +30,14 @@ protected void onTransform(View view, float position) {
 		view.setRotationX(rotation);
 	}
 
+	@Override
+	protected void onPostTransform(View page, float position) {
+		super.onPostTransform(page, position);
+
+		if (position > -0.5f && position < 0.5f) {
+			page.setVisibility(View.VISIBLE);
+		} else {
+			page.setVisibility(View.INVISIBLE);
+		}
+	}
 }