Fix alpha transparency in XCF images

Fix alpha transparency in XCF images

The -layers merge option flattens all layers on to a canvas
of whatever color -background is set to. However -background
was white for the PNG bKGD chunk. Set it to transparent just
for the -layers merge operation.

This works well on most recent image magick. Older image magick
explodes on my test image (I tested 6.6.0-4), so unclear if it
works there, but it certainly didn't make anything worse.
Wikimedia uses a version between the two that I tested so hopefully
this will make things better, and if it doesn't, it eventually will
when we update image magick.

Bug: 35622
Change-Id: I77601cdf005a64ae8ea7516cc846620431917863