Use px size instead of w+h reqs for poster images

The old method worked only by accident on older GTK4 versions.

src/application.css

@@ -7,6 +7,10 @@
 	opacity : 0 ;
 }
 
+.collatable-container flowboxchild {
+    padding : 0 ;
+}
+
 .open-collection-item-button {
 	font-weight : normal ;  /* No bold text by default for this kind of button */
 }