S3DVertex initialize color always. It's derived classes now both accept const S3DVert...

S3DVertex initialize color always. It's derived classes now both accept const S3DVertex& constructor.

color intitialization has some cost, but we initialized all other values already (vectors are always set to 0).
Don't think it's a good idea to have one value around which is random.
S3DVertex2TCoords(S3DVertex& o) to S3DVertex2TCoords(const S3DVertex& o) simply because it makes more sense
S3DVertexTangents(const S3DVertex& o) added because I'll need it later (and no idea why only S3DVertex2TCoords got one of those before).

git-svn-id: svn://svn.code.sf.net/p/irrlicht/code/trunk@6359 dfc29bdd-3216-0410-991c-e03cc46cb475