Mono has removed support for the .NET 2.0 profile, so we'll have to be
using 4.0 for everything next time we want to update.

The refcounting is also tested because some apps depend on that.

When picking a color halfway between 100% white (0xffffffff) and fully
transparent (0x00000000), the result was 50% opaque 50% grey
(0x80808080) when it should really be 50% opaque white (0x80ffffff).
This had a tendency to create grey fringes on things. The fix is to
weight the non-alpha components based on how much they contribute to
the final alpha value.