windowscodecs: WICConvertBitmapSource should ask IWICFormatConverter::Initialize to use an optimized palette.

This matches Windows' behaviour, and makes a simple test application that
converts any WIC compatible RGB(A) source image to 8bppIndexed format work.
Signed-off-by: Dmitry Timoshkov <dmitry@baikal.ru>
Signed-off-by: Vincent Povirk <vincent@codeweavers.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>