[PATCH v2] Implement DwmIsCompositionEnabled

[Anonymous Meerkat]

> Just so you know, DwmIsCompositionEnabled always return TRUE on Windows 8.
> So it shouldn't be that much of a problem only setting *enabled to TRUE.

Thank you, I forgot to check MSDN's documentation before writing the
patch. I've submitted v3.

> You should also consider implementing DwmEnableComposition

According to MSDN however, DwmEnableComposition has no effect under Windows 8?

As for the others, I agree, I'll try to get around to it if I can find
more programs using it to test. Enabling it will cause some programs
to assume various functions will exist, so I think at least stubbing
the DLL is likely going to be important.

Thanks for the review.