fix: Handle calc() with number type in CSS position checking

Bug: A conic-gradient() position containing a calc() that resolves
to a number causes a ClassCastException to be thrown.

Cause: CssCalc.getLength() only handled computed_type == CSS_LENGTH.
When CssBackgroundPosition.checkSyntax() calls getLength() on a
calc with computed_type == CSS_NUMBER, it falls through to the throw.

Fix: Delegate to the inner value’s getLength() when computed_type is
CSS_NUMBER, matching how CssNumber.getLength() handles the zero-check.

Author: sideshowbarker

org/w3c/css/values/CssCalc.java

@@ -410,6 +410,14 @@ public CssLength getLength() throws InvalidParamException {
                 return val2.getLength();
             }
         }
+        if (computed_type == CssTypes.CSS_NUMBER) {
+            if (val1.getType() == CssTypes.CSS_NUMBER) {
+                return val1.getLength();
+            }
+            if (val2.getType() == CssTypes.CSS_NUMBER) {
+                return val2.getLength();
+            }
+        }
         throw new ClassCastException("unknown");
     }