Draw function nexplode was replaced by explode in test case

Draw function nexplode was replaced by explode in test case
blend/complex/D4.
Calculation of gravity center was removed for degenerated edge.
It is revealed that function nexplode does not work on shapes with
degenerated edges.

Comments.
Calculation of gravity center was made by vertices data in case of
degenerated edge without representations especially for test case
blend/complex/D4. The mass of the edge was set to 1. The mass near 0 does
not satisfy the test case. As a result test case bugs/moddata/bug268
became failed. Because only the mass near 0 satisfies last test case.