Render_Contour2: fix incorrect allocation calculation

Render_Contour2: fix incorrect allocation calculation

It is clear from surrounding code that this was attempting to allocate an array
of GLdouble *pointers*, not an array of GLdoubles. It is unlikely this would
have caused any noticeable issues as sizeof(GLdouble) is typically greater than
sizeof(GLdouble*), but nevertheless the calculation was incorrect. This change
not only corrects this, but uses the more appropriate calloc instead of malloc
for array allocation.