WFS layer should remove vectors when redrawing

[crschmidt]

The vector layer currently clears the renderer, but does not clear its internal feature list, so as you zoom in and out, it will continue to draw copies of the features over and over.

[sderle]

add Vector.destroyFeatures() and call it from WFS.moveTo()

[sderle]

I think this will fix the problem... but it needs tests.

[crschmidt]

This won't work because destroy() removes the item from the array. So the loop won't work. I think. Doing the reverse for loop trick should work.

[sderle]

add Vector.destroyFeatures() and call it from WFS.moveTo(), plus tests

[sderle]

take two, including tests

[crschmidt]

Looks good. Do it to truett.

[sderle]

r2996