OLEAUT32: transparency fix for OLEPicture

[Alex Villaci­s Lasso]

 > The bad news is that this fix uncovers an even greater bug: builtin 
oleaut32 is unable to create a valid AND mask
 > for a bitmap with transparency. The end result is that the AND mask 
(which is now created in the first place)
 > holds a garbage bitmap, rather than a valid outline of the sample image.

Well, I figured out why the AND mask was garbage - the bitblt operation 
was using the width and height from the bitmap structure used to create 
the DIB bitmap, without considering that the height had a negative sign 
(to account that a Windows DIB fills scans in the reverse order from a 
GIF). Therefore the AND mask was copied to an area completely outside of 
the target area for the temp HDC, so the target area remained unchanged 
(and therefore undefined).

Then I had this problem that, once the image was being rendered as 
transparent, the transparent areas showed my desktop below the sample 
window. This indicated that the transparent bitmap was not being 
announced as transparent by OLEPicture. This was a trivial fix in 
get_Attributes(), so that now the window background was rendered before 
the bitmap was blasted.

Third, the transparency was (incorrectly) selecting as transparent some 
areas that were not meant as transparent in the original GIF. The root 
cause was that the algorithm used to build the AND mask was relying on a 
RGB value extracted from the transparency index, which is incorrect - 
there is no guarantee that the RGB value pointed to by the transparency 
index is not being used by any other opaque palette index. So the AND 
mask building was rewritten to use the transparency index directly.

Changelog:
* Fix leftover negative sign in height parameter for transparent bitmap
* Properly announce whether bitmap is transparent in get_Attributes
* GIF transparency is now palette-index based, instead of RGB based
* Keep original bitmap and XOR mask separate, so that get_Handle returns 
original bitmap

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