Added the set methods for Vec4 types.

There are two set method variants for Vec2 and Vec3, but no corresponding methods for Vec4.
Added setter methods for Color and ColorA that are consistent with many other basic types.
SvgGl.h now includes gl/Texture.h which is required by the SvgRendererGl::drawImage method.

It is possible to use the SvgRendererGl without having previously included gl/Texture.h. This will prevent a compiler error in that case.
Merge pull request #308 from calebjohnston/Vector-Color-additions

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