windowscodecs: Implement Palette_InitializeFromBitmap.

[Vincent Povirk]

>> This does not agree with MSDN's description of the function.
>>
>> According to MSDN, InitializeFromBitmap should compute a palette based
>> on the contents of the (presumably) non-paletted bitmap data.
>
> For a bitmap with palette my implementation should be good enough.

For a bitmap with a palette you can easily end up with too many
colors, and no transparent color when one is requested. It doesn't get
us any closer to a real implementation, and if we had one it would
always be preferable this one. That case of a bitmap with a palette is
exactly when the method is least likely to be used, so mostly you're
just leaving the problem and making it harder to identify the cause.