Pica/Math: Improved the design of the Vec2/Vec3/Vec4 classes and simplified rasterizer code accordingly.

- Swizzlers now return const objects so that things like "first_vec4.xyz() = some_vec3" now will fail to compile (ideally we should support some vector holding references to make this actually work).
- The methods "InsertBeforeX/Y/Z" and "Append" have been replaced by more versions of MakeVec, which now also supports building new vectors from vectors.
- Vector library now follows C++ type promotion rules (hence, the result of Vec2<u8> with another Vec2<u8> is now a Vec2<int>).