LibWeb: Use rect value in CSS clip property

When a rect value is passed to the clip property via CSS, keep it in
ComputedValues so that at a later stage can make use of it.

Userland/Libraries/LibWeb/CMakeLists.txt

@@ -25,6 +25,7 @@ set(SOURCES
     Crypto/Crypto.cpp
     Crypto/SubtleCrypto.cpp
     CSS/Angle.cpp
+    CSS/Clip.cpp
     CSS/CSSConditionRule.cpp
     CSS/CSSGroupingRule.cpp
     CSS/CSSImportRule.cpp