FIx PySequence_AsVectorCoords and discourage usage

[ankith26]

Whenever is PySequence_AsVectorCoords is called without calling pgVectorCompatible_Check beforehand, currently the code gives undefined behaviour when expected dim is 3 and a Vector2 is passed. If expected dim is 2 and Vector3 is passed the 3rd component gets ignored. For now I fixed the undefined behaviour case to ensure consistent 0-ing.

I renamed this function to pg_VectorCoordsFromObjOldDontUseInNewCode so that new API doesn't end up using this function. New API should just use pg_VectorCoordsFromObj directly.

Wherever pgVectorCompatible_Check was already being called before PySequence_AsVectorCoords we can safely use the new pg_VectorCoordsFromObj API