Add SLERP to Vector{2,3}, optimize Quat's Vector3 rotation.

Also even out Basis and Quat APIs a little.

core/math/matrix3.cpp

@@ -826,3 +826,16 @@ void Basis::set_diagonal(const Vector3 p_diag) {
 	elements[2][1] = 0;
 	elements[2][2] = p_diag.z;
 }
+
+Basis Basis::slerp(const Basis &target, const real_t &t) const {
+	// TODO: implement this directly without using quaternions to make it more efficient
+#ifdef MATH_CHECKS
+	ERR_FAIL_COND_V(is_rotation() == false, Basis());
+	ERR_FAIL_COND_V(target.is_rotation() == false, Basis());
+#endif
+
+	Quat from(*this);
+	Quat to(target);
+
+	return Basis(from.slerp(to, t));
+}