[PATCH v4 1/2] windowscodecs: Use IWICImagingFactory_CreatePalette in write_source.

[Esme Povirk (they/them)]

For some reason that discussion never got to my mailbox.

I don't like duplicating the file because the interface may change, or
the wrappers on the unix side may become more complex. Perhaps
encoder_create/decoder_create could be moved somewhere else to avoid
an ifdef, but there doesn't seem to be a good place to put it.

I'll have to think about this.