Fix CSS offset path animations

Improving parsePath function 79d9c7a67654c07ae71ac589da326d1db7fa9e03
means that QGuiSvg::parsePath will reject the value from m_offsetPath
as it is in the form of path("<path>"). Capture the path string first
before passing it to QGuiSvg::parsePath

Pick-to: 6.12
Change-Id: I12430b2824ac4f669ae4a426443b8684995c4798
Reviewed-by: Eirik Aavitsland <eirik.aavitsland@qt.io>