css: do not stop processing when finding a custom property.

If a user agent finds something that it does not support, should ignore it and
proceed. See https://www.w3.org/TR/CSS22/syndata.html#parsing-errors

"User agents must ignore a declaration with an unknown property."

See also #35.

echosvg-css/src/main/java/io/sf/carte/echosvg/css/engine/CSSEngine.java

@@ -1718,7 +1718,7 @@ public void property(String name, LexicalUnit value, boolean important) {
 
 		@Override
 		public void lexicalProperty(String name, LexicalUnit value, boolean important) {
-			throwUnsupportedEx();
+			// Ignore custom properties
 		}
 
 		private void throwUnsupportedEx() {