Transparency error when converting a png to bmp

When converting a png icon with transparent background to bmp, I got a
image with black background and the actual icon became transparent...

You can download this image
http://www.iconarchive.com/show/I-like-buttons-3a-icons-by-mazenl77/Cute-Ball-Go-icon.html,
and run

gm convert Cute-Ball-Go-icon.png c.bmp

with gm 1.3.21 and check the result bmp.

I found that bug #271 fix a problem which cause a blank result when
converting bmp to jpg. The fix reverse the opacity of a pixel in bmp
decoder. So I tried to also reverse the opacity of a pixel in bmp encoder.

Modify coders/bmp.c, change line 1736 from
*q++=ScaleQuantumToChar(p->opacity); to
*q++=MaxRGB-ScaleQuantumToChar(p->opacity);

It fix my problem but I'm not sure whether it is the right approach.

Thanks.

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