gdiplus: Use StretchBlt instead of GdiAlphaBlend if device doesn't support alpha blending.

[Vincent Povirk]

>> > I tested printing quite a bit of different documents with an application
>> > I have here and all of them get printed correctly with the patch. Before
>> > the printed pages were just blank. What do you think is better for a user?
>>
>> It's not good if things work by coincidence in one case that's
>> important to you, and then we can't fix other cases later (or we can't
>> fix other cases without breaking yours).
>
> I don't understand your objection, nothing prevents you from improving
> my patch. Printing something is still better than printing nothing, consdier
> this as adding a partial implemenation in the place of a stub. GdiAlphaBlend
> is not supported by printers, that's a matter of the fact, using StretchBlt
> for now is better than doing nothing and even not printing a FIXME or returning
> an error to an application. Feel free to implement it differently, until that
> my implementation is suffice for many applications.

Printing something is worse than printing nothing when it's a giant
black box that happens to contain the thing that was supposed to draw,
and that will start happening if you make this change. And then I
won't know how to fix it.