windowscodecs: Move JPEG frame image data initialization from Frame::CopyPixels to Decoder::Initialize.

This is how PNG decoder does things, and it avoids image data corruption
in some cases (presumably when libjpeg reuses existing scanline data).
Signed-off-by: Dmitry Timoshkov <dmitry@baikal.ru>
Signed-off-by: Vincent Povirk <vincent@codeweavers.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>