gdi32/tests: Some GdiAlphaBlend tests

[Nikolay Sivov]

Roderick Colenbrander wrote:
> Regarding the zero source DC, what happens visually?
>
> On Wed, Sep 9, 2009 at 1:30 PM, Nikolay Sivov<bunglehead at gmail.com> wrote:
>   
>> Bug http://bugs.winehq.org/show_bug.cgi?id=18041 shows this commented crash.
>> I'm not sure how to fix it, but I think this call can't do anything
>> without source context.
>>
>> Another question is where to place such check - in gdi32 or winex11...any
>> advices are welcome.
>>
>> Changelog:
>>    - Some GdiAlphaBlend tests
>>
>> >From a59cadcf03edeefff95c340230a47c9c3c60727f Mon Sep 17 00:00:00 2001
>> From: Nikolay Sivov <bunglehead at gmail.com>
>> Date: Wed, 9 Sep 2009 15:20:34 +0400
>> Subject: Some GdiAlphaBlend tests
>>
>>     
I tried with:
---
    blend.BlendOp = AC_SRC_OVER;
    blend.BlendFlags = 0;
    blend.SourceConstantAlpha = 200;
    blend.AlphaFormat = 0;
---
in a simple mfc project to bitblt between two buttons. Using NULL source 
visually does nothing. Maybe this could change in future
but currently with the only AC_SRC_OVER operation available it does 
nothing without source.