QImage/QPixmap: use rvalue overloads more
to reuse internal buffers
See also https://github.com/qt/qtdeclarative/commit/ff0cbaeba3dfc11dde5445b9ea919f1ae137ea2f
to reuse internal buffers
See also https://github.com/qt/qtdeclarative/commit/ff0cbaeba3dfc11dde5445b9ea919f1ae137ea2f