Support vector center in linear_extrude, only z matters.

You can now use a vector center argument for linear extrude:
    linear_extrude(..., center=[false, true, true], ...);
but only the z argument is used (like cylinder vector center).

Using center=true|false still works as before.

Added test case.
Support vector center in surface

You can now use a vector center argument for linear extrude:
    surface(..., center=[false, true], ...);
for individually centering on the x and y axes.

Using center=true|false still works as before.