QWebpHandler: make m_webpData a non-member

The documentation of WebPDemux() is silent on the required lifetime of
its argument, but the implementation just copies the values instead of
storing a pointer. This may be obvious to a C programmer, but I guess
the pass-by-pointer is done to allow the struct to grow without changing
the library entry point.

So make m_webPData a local variable and not a data member, and scope
the lifetime extra tight (IILE), so that tools can alert as to any
lifetime issues if and when the implementation changes (I don't think
it can, this is Google code, and Google is also home to Hyrum, of
Hyrum's Law, which binds the implementation now).

Still, not picking back too much, as it's not totally risk-free.

Pick-to: 6.12
Change-Id: I5530e298e09dae326f915fd91afe596888d9e2b5
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>