QWbmpHandler: add a missing std::move() in read()

Pointed out by Coverity.

Use this opportunity to get rid of the old-fashioned Hungarian/SESE
style in the function.

Amends the addition of the wbmp plugin to this module
(aa48df6f9b4a431b3a802ac1a2a15db1ed5dc696, 5.0).

Pick-to: 6.12
Coverity-Id: 426868
Change-Id: I5df57a87bfc59eb33d93219ff99cc44f1731448e
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>