Video core: Add utility class for vector operations.

I wrote most of this for ppsspp, so I hold full copyright over it.
In addition to the original release in ppsspp, this provides functionality to easily extend e.g. two-dimensional vectors to three-dimensional vectors.
Pica: Add float24 structure.

24-bit floating points are used internally for calculations on the GPU, however the current code will still emulate that with 32-bit floating points.
In the future we might want to accurately perform the calculations with correct bitness in the future, but for now we just wrap the calculations around this class.
Pica: Add command processor.
Pica: Add register definition for vertex loading and rendering.
Pica: Implement vertex loading.
Pica: Add vertex shader implementation.
Pica: Add primitive assembly stage.
Pica: Add triangle clipper.
Pica: Add basic rasterizer.