Fix bug in prepFillArea.
Optimize prepFillArea.

Rather than allocating (and rendering!) a unique polydata + mapper + actor
combo for each vertical slice of the color bar, create a single, multicell
polydata that gets rendered at once in a single actor/mapper.

This speeds up the execution of prepFillArea using the test script in #849
from 0.25s to 0.09s on my machine.
Merge branch 'master' into 849_prepFillAreaCleanup
Merge pull request #877 from UV-CDAT/849_prepFillAreaCleanup

849 prep fill area cleanup