[lazarus] TGraphic Transparency

[andrew johnson]

I would like to get TGraphic Transparency working, and since it only
affects display, and that is always handled by StretchBlt in the long
run, I was hoping to add it there. 

However since png's, and other more advanced graphic types can have an
existing transparency for instance, I would like to get the general
opinion on exactly how this should be handled. With XPM's/BMP's for
instance, I believe it is simply whatever is the current brush color is
replaced with a transparent mask, but I am not positive as it has been
awhile since I used this feature myself. 

So anyway, essentially I have the following questions before I finish
and commit:

1.) How should transparency be handled with graphic types that support
it natively when

   a.) the current graphic has such a channel
   b.) it doesn't

2.) how should it be handled on types which don't support it natively

Andrew