webp handler: Avoid uninitialized data in animation frames

Since webp animation frames may optionally cover only parts of the
image area, ensure that the whole QImage is initialized. This was
earlier done only for image formats with transparency.

This also fixes a shortcoming where uncovered areas in opaque formats
where expected to be filled with the defined background color.

Task-number: QTBUG-148332
Pick-to: 6.12 6.11 6.8 6.5 5.15
Change-Id: Ic606468cc2d30a31b65884c39bb80181d7e3aeb3
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@qt.io>