webp handler: support alpha-less reading and writing

Webp files can be with or without alpha channel. The handler would
ignore this and read all as Format_ARGB32 images, and write all as
having alpha, in both cases losing that important bit of information.

As a driveby, simplify the endianness handling in write(). By always
converting the source image to an endianness-independent QImage
format, no special handling is required.

Task-number: QTBUG-48628
Change-Id: I624ed72b18a8b59a542979efcb4e8ff81214e0d7
Reviewed-by: Liang Qi <liang.qi@qt.io>