Currently the server treats incoming buffers as not premultiplied.
KWayland::Client sends data that is ARGB32 and ARGB32_Premultiplied as the same
WL_SHM_FORMAT_ARGB8888.
According to a post on wayland-devel by Fredrik Höglund, all RGB data
should be treated as premultiplied, which matches what Qt is doing.
I've changed client to perform a conversion rather than sending
mismatched data, but this commit will still breaks a bunch of tests in
the stable kwin branch as it compares the server output to a fixed
QImage with a format.