Support alpha channels in model patterns

Uses the alpha channel in the pattern texture to interpolate the pattern tint from white to the normal pattern colour.

![](http://i.imgur.com/gRiifYX.png)

Test pattern used above:

![](http://i.imgur.com/8fAovR8.png)

I don't really know what I'm doing when it comes to shaders, so hopefully I'm not doing anything stupid there.