wined3d: fix check for P8 render target

[Alexander Dorofeyev]

Yes your patch fixes most problems (there are some remaining issues but they 
were not fixed by mine either).

Roderick Colenbrander wrote:
>> Hi.
>>
>> Yes, exactly, the AVP1 game is using p8 textures and color keys on some of
>> them
>> for transparency.
>>
>> The thing is, the "index_in_alpha" hack DOES get enabled on its P8 3D
>> textures,
>> and that's the problem I'm trying to fix. I don't see the "primary render
>> target
>> is p8" check in the code, instead it looks like it checks that
>> render_targets[0]
>> is a render target.
> 
> Try to see if the attached patch fixes the problem. It seems that at some point I didn't check whether the target is P8 in this function while I check for it in other places. I have checked it using all my p8 testcases and it seems to work fine but I don't have AVP. If it works I'll submit it.
> 
> Roderick
> 
> 
>