More warning cleanups

 - Add `#pragma warning` to dyncom and skyeye stuffs to ignore their warnings on MSVC
 - Add a `ToBool()` method to `BitField` to avoid manually/implicitly casting to bool
There seems to be a bug in VS with `explicit operator`:

http://stackoverflow.com/questions/20498142/visual-studio-2013-explicit-keyword-bug
https://connect.microsoft.com/VisualStudio/feedback/details/811334/bug-in-vs-2013-support-for-explicit-conversion-operators

It fails to compile on VS2013 too:

    viewport.offset_x   = float24::FromFloat32(static_cast<float>(registers.viewport_corner.x));

>6>..\..\..\src\video_core\clipper.cpp(89): error C2440: 'static_cast' : cannot convert from 'BitField<0,16,u32>' to 'float'
>6>          Ambiguous user-defined-conversion

I think what happens here is that we are trying to cast a `BitField` to `float`, and since VS doesn't honor the `explicit`, it thinks that it is also acceptable to implicitly call `bool()` before casting.

Anyway, I'll just stick with `ToBool()` for now, and I'll add a note to it.