Simple Vector and Color method additions. Minor fix for SvgGl.h.

* Added Vec4::set methods that behave like the Vec2::set and Vec3::set methods. 
* Added corresponding set methods for Color or ColorA values. 
* Added another header #include directive for SvgGl.h for gl/Texture.h. Without it, a compiler may throw an error if the Texture header has not been previously included by another file.