New dissolve code

[Fred Kiefer]

Hi Adam,

there seems to be a problem with the new code you added for the
dissolving of images. When running the Composite test in GSTest there is
no image displayed any more. This might either be a problem in the code
of GSTest or more likely a problem in the dissolve code. So my question
is if you could give a bit of explanation what this code is doing. I had
a short look at the change, but failed to get the idea behind the code.
For one side the X pixel gets multiplied by the fraction and for the
other image only the alpha value is multiplied. As I never really did
understand what dissolve should do this doesn't make it any clearer for
me.

Cheers
Fred